There's several common threads in the info below - Robert J. Horner's brothers were William and Edmund/Edmond and I've highlighted them in this COLOR. Horner's birth year was 1893, 1894, 1895 or 1896, and I've highlighted those variations in this COLOR.

• Cook County, Illinois marriage index for the January 26, 1890 marriage of 23 year old Daniel or David Horner and 20 year old Maggie Kleine at St. Patrick's Church, Chicago, Cook County, Illinois:
  https://www.familysearch.org/ark:/61903/1:1:N76C-YLF
  https://www.familysearch.org/ark:/61903/1:1:V2P6-6CG
• 1900 census: living in Spring Valley, Illinois are 34 year old Daniel Horne [sic] (born Ireland; occupation "Coal miner"), 30 year old wife Margaret (born Illinois), 9 year old daughter Cecil [sic] (born Illinois), 7 year old daughter Glena (born Illinois), 6 year old son Robert (born Illinois, September, 1893), 4 year old son Willie (born Illinois), and 2 year old son Edmond (born Illinois): https://familysearch.org/pal:/MM9.1.1/MSSW-66J
• 1910 census: 16 year old Robert Horner and 14 year old William Horner are among dozens of teenaged boys (listed as "lodgers") at the "Working boys Home" at 1128-1140 W. Jackson Boulevard, Chicago. The occupation of Robert is "Errand - boy" and William is an "Apprentice - printer" (and those are the occupations listed for most of the boys at the home): https://familysearch.org/pal:/MM9.1.1/MKZH-ZXW?cc=1727033
  This is the Mercy Home for Boys and Girls in Chicago. The home is still in operation and the following link will take you to their history page - their Headquarters is at 1140 W. Jackson Blvd., just west of downtown Chicago: http://www.mercyhome.org/mercy-home-history
• World War I draft registration dated June 5, 1917 for 22 year old Robert John Horner, 4040 Broadway Avenue, Chicago. He is single; born September 14, 1895 in Spring Valley, Illinois; occupation is "paper writing, North Shore Review" at 4040 Broadway Avenue, Chicago; and the registration notes "Physical disability - limbs off" and "Limbs severed": https://familysearch.org/pal:/MM9.3.1/TH-1951-26328-17264-36?cc=1968530
• More on his 1923 marriage is further down this webpage.
• 1930 census - owning their home at 425 North Harper Avenue, Los Angeles are 32 year old Robert J. Harner [sic] (born Illinois) and his 29 year old wife Frieda (born Missouri). Also living there is 21 year old brother-in-law Julius Bohnn (born Missouri). No occupations listed for any of the three people: https://www.familysearch.org/ark:/61903/1:1:XCV7-C3M
• 1942 World War II draft registration dated April 25, 1942 for 48 year old Robert John Horner - he was born September 14, 1894 in Springvalley, Illinois; occupation is "Road Show Exhibitor"; and his home address is 962 N. Wilton Place, Los Angeles. He is approximately 3 feet tall, 122 pounds, and is legless. His contact is Wm. T. Horner, 5937 N. Maplewood Ave., Chicago, Ill. (his brother who is mentioned in the funeral notice below). There's a handwritten notation on the registration card: "cancel - deceased 7-29-42 El Paso, Texas": https://familysearch.org/pal:/MM9.3.1/TH-1-16386-40835-52?cc=1861144
• Texas death certificate for Robert J. Horner - he was born September 14, 1896 in Illinois; is divorced; occupation is "Projectionist"; and his parents were Dan Horner and Margaret Cline. He passed away on July 29, 1942 at the El Paso City-County Hospital, and cause of death was cirrhosis of the liver. Death certificate notes that he was living in Clint, El Paso County, Texas, a small town about 25 miles southeast of the city of El Paso. Interment at Evergreen cemetery (no city mentioned for the cemetery, but I assume it was El Paso): https://www.familysearch.org/ark:/61903/1:1:K3DS-L5L
• There was a death notice and funeral information in editions of the El Paso (Texas) Herald Post newspaper:
  Excerpts from July 30, 1942 death notice: "Funeral arrangements for Robert J. Horner of Hollywood, Cal are pending ... died yesterday in a hospital ... survived by two brothers, William and Ed Horner of Chicago, Ill."
  Excerpt from the August 1, 1942 funeral mention: "Funeral services ... were held at ... Kaster and Maxon Chapel ..." ; "Burial was in Evergreen Cemetery."

Newspapers and trade publications had reports on Horner's 1923 marriage (and divorce, according to his death certificate). But there are variations with his wife's first and last name as well as their marriage location.

• September 27, 1923 Spring Valley Illinois Gazette - headline: "FORMER VALLEY BOY, LEGLESS FILM GENIUS, IS MARRIED." ; Article excerpt: "Robert J. Horner, at one time a resident of Spring Valley (Illinois), legless motion picture producer, and Miss Frieda Bohn of Chicago, his boyhood sweetheart, were married ..."
• September 27, 1923 Variety noted Horner's Los Angeles marriage to Freda Bohm of Chicago, and "... losing his legs in an accident when six years old."
• October 18, 1923 Variety had a lengthy article covering St. Louis news: "Frieda Bohnn, of this city, was married to Robert J. Horner, a legless movie director, of Los Angeles, Sept. 17 (1923). News articles of the marriage give Miss Bohnn's address as Chicago. That is because she was visiting friends in Chicago at the time she became acquainted with Horner, who lived there until two years ago. The bride's mother lives in St. Louis and is owner of Longwood Wine Garden in Carondelet."
• October 13, 1923 Motion Picture News: "... marriage of Frida A. Bohnn, St. Louis girl, to Robert J. Horner, Hollywood film director, at Tia Juana, Mexico, with Eva Novak and George Cresebro [sic] as witnesses." ; "Miss Bohnn is the daughter of Mrs. Anna C. Bohnn ... St. Louis."

Was Frieda / Frida's last name Bohn, Bohnn, Bohm? Based on records for her brother Julius, the family surname appears to be BOHNN.

• 1910 census - living at 9406 South Broadway, Carondelet township, St Louis County, Missouri are 35 year old Julius Bohm (born Missouri; occupation "Garden - Wine"), his 33 year old wife Anna (born Missouri), 9 year old daughter Frieda (born Missouri), 6 year old daughter Henrietta (born Missouri), 1 year old son Julius (born Missouri), 71 year old mother Anna Bohm (born Germany), 22 year old brother-in-law Max Clausen (born Missouri), and a servant: https://www.familysearch.org/ark:/61903/1:1:M219-HJW
• 1920 census - owning their home at 9406 South Broadway, Carondelet township, St Louis County, Missouri are 43 year old Anna Bohman (born Missouri; occupation "Farmer - grape farm"; widowed), 18 year old daughter Frieda Bohman (born Missouri about 1902), 15 year old daughter Henritta [sic] (born Missouri), and 11 year old son Julius (born Missouri): https://www.familysearch.org/ark:/61903/1:1:M8Z9-WZP
• 1930 census - owning their home at 425 North Harper Avenue, Los Angeles are 32 year old Robert J. Harner [sic] (born Illinois) and his 29 year old wife Frieda (born Missouri). Also living there is 21 year old brother-in-law Julius Bohnn (born Missouri; single). No occupations listed for any of the three people: https://www.familysearch.org/ark:/61903/1:1:XCV7-C3M
• Marriage license for 21 year old Julius Henry Bohnn (born Missouri) and 21 year old Lucille Ethyln Brown on June 1, 1930 in Los Angeles. First marriage for both. His occupation is "Selling - Tire Business". Parents were Julius Henry Bohnn and Anna Clausen: https://www.familysearch.org/ark:/61903/1:1:K8NB-GV1
• 1940 census - renting in Waco, McLennan County, Texas are 32 year old Julius Bohnn (born Missouri; occupation "Salesman - Wholesale Tire Co."), his 27 year old wife Gisella (born Missouri), and 4 year old son Julius (born Texas): https://www.familysearch.org/ark:/61903/1:1:K4SS-5P7
• Texas death certificate for Jules Bohnn, born September 3, 1909 in St. Louis, Missouri, parents were Jules Bohnn and Anna Clawson, and he passed away June 26, 1961 in Wichita Falls, Wichita County, Texas: https://www.familysearch.org/ark:/61903/1:1:K3HH-N6P

Robert J. Horner's parents, step-father, siblings.

• Cook County, Illinois marriage index for the January 26, 1890 marriage of 23 year old Daniel or David Horner and 20 year old Maggie Kleine at St Patrick's Church, Chicago, Cook County, Illinois:
  https://www.familysearch.org/ark:/61903/1:1:N76C-YLF
  https://www.familysearch.org/ark:/61903/1:1:V2P6-6CG
• 1910 census for Horner's mother Margaret and daughter Cecile. 37 year old Margaret (born Illinois) has re-married and is now Margaret C. Milliken and her husband is 35 year old William Milliken (born Scotland). 17 year old daughter Cecile M. Horner (born Illinois) is living with her along with new daughter Olive M. Milliken: https://www.familysearch.org/ark:/61903/1:1:MKZ8-MTC
• 1920 census - living at 4040 Broadway, Chicago, Cook County, Illinois are 46 year old William Milliken (born Scotland; occupation "Engineer - Packing House"), his 44 year old wife Margaret Milliken (born Illinois), and 10 year old daughter Olivet (born Illinois). Also living with them are: 23 year old William Horner (born Illinois; occupation "Printer"), 21 year old Edwin [sic] Horner (born Illinois; occupation "Printer"), and 19 year old son Rosemunt Horner (born Illinois). I think the Rosemunt / boy is really daughter Rose / Rosamond: https://www.familysearch.org/ark:/61903/1:1:MJ79-2K9
• 1930 census - living in Chicago, Cook County, Illinois are 58 year old William Milliken (born Illinois), his 56 year old wife Margaret Milliken (born Illinois), 20 year old daughter Olivet Milliken (born Illinois), and 22 year old stepdaughter Rose Horner (born Illinois): https://www.familysearch.org/ark:/61903/1:1:XS52-DL5
• Illinois Death Index for 69 year old Margaret M. Milliken - father's name was Bernard Cline; husband was William A. Milliken; and she passed away on February 10, 1939 in Chicago, Cook County, Illinois: https://www.familysearch.org/ark:/61903/1:1:NQSB-T6P
• ProQuest obituaries had a funeral notice in the February 12, 1939 Chicago Illinois Daily Tribune for Margaret M. Milliken, nee Cline. Survivors were husband William; and she was the mother of Robert, William, Edmund Horner, Cecil [sic] Murphy, Blanche Swift, Rosamond Wagman, and Olivette Hasvold.
• 1940 census - rooming in Chicago, Illinois is 67 year old William Milliken (born Scotland), and his occupation is "Stationary engineer - Building Maintenance": https://www.familysearch.org/ark:/61903/1:1:K497-467

More on Robert J. Horner's brothers, Edmund and William.

• 1930 census - and brothers Edmund and William are living a couple doors away from each other in Chicago and both are still in the printing business:
  renting at 2554 Fitch, Chicago, Illinois are 33 year old William T. Horner (born Illinois; occupation "Compositer - Printing") and his 29 year old wife Alice: https://www.familysearch.org/ark:/61903/1:1:XS5G-ZV6
  renting at 2556 Fitch, Chicago, Illinois are 30 year old Edmund Horner (born Illinois; occupation "Salesman - Printing"), his 26 year old wife Mildred (born Illinois) and 2 year old son Edmund (born Illinois): https://www.familysearch.org/ark:/61903/1:1:XS5G-ZVG
• 1940 census for Horner's brother Edmund - renting at 6321 Talman Avenue, Chicago are 41 year old Edmund D. Horner (born Illinois; occupation "Printing Salesman"), his 36 year old wife Mildred L. (born Illinois), 12 year old son Edmund D. (born Illinois) and 7 year old daughter Patricia (born Illinois): https://www.familysearch.org/ark:/61903/1:1:K49H-TMQ
• 1942 World War II draft registration for Horner's brother William Thomas Horner - he lives at 5937 Maplewood Ave., Chicago; was born 1896 in Spring Valley, Illinois; and his contact is brother E. D. Horner, 6321 N. Talman Ave., Chicago: https://familysearch.org/pal:/MM9.3.1/TH-266-11820-26004-93?cc=1861144
• Cook County, Illinois death index for 54 year old William Thomas Horner who passed away April 21, 1951 in Chicago, Cook County, Illinois. He was born August 24, 1896 in Illinois; parents were Daniel Horner and Margaret Cline; and his occupation was "Printer": https://www.familysearch.org/ark:/61903/1:1:Q2M4-LSCT
• Cook County, Illinois death index for 67 year old Edmund D. Horner who passed away April 11, 1966 in Chicago, Cook County, Illinois. He was born August 20, 1898 in LaSalle, Illinois; parents were Daniel Horner and Margaret Cline; and his occupation was "Printer": https://www.familysearch.org/ark:/61903/1:1:Q2MX-6MT1

Robert J. Horner Links:

  Although some of the data may be incomplete or inaccurate, the Internet Movie Database (IMDb) has information on Horner: http://www.imdb.com/name/nm0395130/

History of the short-lived Ebony Film company of Chicago: http://normanstudios.org/blog/2017/01/17/the-rise-and-fall-of-ebony-films/

You can download/stream several of Horner's western epics from YouTube:

There's several clips of the silent THE WHITE OUTLAW (1929) which stars Art Acord and features Al Hoxie as a lawman and Lew Meehan as an outlaw. Opening titles and credits are Ralph M. Like's Exhibitors Film Corporation and direction by Robert J. Horner: https://www.youtube.com/results?search_query=%22white+outlaw%22
Jack Perrin in the silent SHOOTIN' SQUARE (Anchor, 1924): https://www.youtube.com/watch?v=M31YzA5yFdU
Ted Wells and Jimmy Aubrey in THE PHANTOM COWBOY (Aywon, 1935): https://www.youtube.com/results?q=%22PHANTOM+COWBOY%22&sp=EgIYAg%253D%253D
Jack Perrin in THE APACHE KID'S ESCAPE (Cosmos/West Coast, 1930): https://www.youtube.com/results?sp=EgIYAg%253D%253D&search_query=%22APACHE+KID%27S+ESCAPE%22
BORDER MENACE (Aywon, 1934) with Jimmy Aubrey as Bill Cody's sidekick "Polecat Pete": https://www.youtube.com/results?search_query=%22BORDER+MENACE%22

Oldies.com has DVDs of a few surviving Horner silent westerns. Heroes are Art Acord, Al Hoxie, and Pawnee Bill Jr. (Ted Wells). Probably can also find these at Amazon and other video sources: https://oldies.com/search/results.cfm?q=%22robert+j.+horner%22&results=tags_DVDs

On Saturday night, May 4, 1935, there was a car accident near San Diego which claimed the lives of four of five people. Child star Jackie Coogan was injured but lived. Coogan's father, John H. Coogan, was the driver and died at the scene. Also killed were 19 year old Trent 'Junior' Durkin, Charles (or Clarence) Jones (the manager of the Coogan Ranch), and Robert J. Horner. Newspaper clippings mention that this Robert J. Horner was an "actor ... 25 years of age ... a young screen and radio writer". Since Jackie Coogan's father was the driver, two lawsuits were filed seeking financial compensation from Jackie - one was filed by Junior Durkin's family and a second by Horner's mother, Anna V. Horner.

A consistency across these records and newspaper articles is his mother, Anna V. Horner:

• 1910 census: 28 year old Robert J. Hoerner [sic] (born New York), 26 year old wife Anna V. (born New York), 3 year old daughter Elsie V. (born New York), 2 year old daughter Muriel E. (born New York), and newborn son Robt J. (born New York) are living in Queens, New York. The occupation of father Robert J. Hoerner is "Merchant - Furniture". The census visit occurred on May 21, 1910 and newborn son Robert J. is 11/12 old (indicating about eleven months of age, which would put his birth year as 1909): https://familysearch.org/pal:/MM9.1.1/M53Y-QYP
• 1930 census: renting at the Hotel Dauphin, 67th Street and Broadway, New York City are 46 year old Anna Horner (born New York; widowed), 23 year old daughter Elsie V. (born New York), 22 year old daughter Muriel (born New York), 20 year old son Robert J. (born New York) and 18 year old daughter Constance (born New York). Occupation of 20 year old Robert J. Horner is "Stenographer - Chemical": https://familysearch.org/pal:/MM9.1.1/X4KG-XNS
• Newspapers indicate that the Robert J. Horner killed in the vehicle accident was 25 years old (which would put his birth date as about 1909). There was a funeral notice for Robert J. Horner in the May 7, 1935 Los Angeles (California) Times newspaper. Excerpt: "Robert J. Horner, beloved son of Anna V. Horner, loving brother of Muriel, Constance and Elsey V. Horner." (The mother and three sisters mentioned in that funeral notice match the 1930 census info above.)
• The California Death Index for years 1909-1939 doesn't provide much detail - there is a record for Robert J. Horner passing on May 4, 1935. He was born about 1910 and was about 25 years old: https://familysearch.org/ark:/61903/1:1:QKS9-35FY.
  No info in the Social Security Death Index (SSDI) since this Robert J. Horner died prior to the issuance of Social Security numbers.
• Anna V. Horner's lawsuit against child actor Jackie Coogan ended in June, 1936, and there were newspaper reports. Excerpt: "Judge Swain ... instructed the jury to return a verdict for the defendant on the ground that a minor has no control over the action of his father and hence young Coogan was not responsible for Horner's death." (The suits by the Durkin family were also dismissed.)
• His mother, Anna V. Horner, is specifically mentioned in the accident articles as well as many newspaper reports on her lawsuit against Jackie Coogan. The California Death Index has a record on Anna V. Horner, born October 12, 1886 in New York and passed away on February 26, 1973 in the Los Angeles area.