So, the surviving children (other than Robert junior), from first wife, Elizabeth, were with Robert and second wife Esther at 1921, suggesting Robert's RAF record from 1918/19, which stated he was a widower, was correct, and as it apparently also stated this on his marriage to Esther, all seems in order, yet still no obvious death record for Elizabeth in Scotland, despite it likely occuring between 1911 and 1919.
frederickay, what was Elizabeth's mum's surname, per the 1901 marriage?