Sheila's grandson can track Colin. Apparently he went to England under a financial cloud. And signature was different. 
Your efforts are immensely appreciated.
What happened to this Colin? Did he have any children? When and where did he die?
Searching NSW bdm 1880-1900, I found no births for Robert, Sheila, Colin, or Hilda with parents Walter and Kathleen....no births in NSW to a Walter and Kathleen with a surname beginning with 'S' in that time frame.
What are the odds of finding a family with a connection to British India, in the right time frame, including Walter (with a surname S, who just happens to be a photographer), wife Kathleen (nee Johnston/e) with children Sheila, Colin, Hilda. Seems highly unusual to have all those people connected in the one household...the only thing missing is proving a connection to Robert.
Every avenue has lead to more questions and mysteries, like Robert Richardson Seale or birth for Robert Edward Seale 1889 who married in 1920. No leads yet either on the older Robert Edward Seale/ Sabinan (Madaru) line.
Holding on to a whole bunch of straws, is certainly a lesson in persistence and patience in an effort to leave no stone unturned
