Not much has been found on Robert Mullins to date.
To muddle things a bit, the item below from the Victoria Advocate, 11 Mar 1899 shows a Robert Mullins selling his photography business in Victoria, TX, to Mr. Rodman. (Note: The printing is obscured a bit. Perhaps the name on this one is Robert Mailins.)
The small town of Victoria, TX, is NOT close to the towns of Marlin and Kosse, TX where JOHN Mullins was working.
No research on Kosse of Texas at this time.