Hi Peter
I don't know about Doris having a son Mervin, but it seems she may have had a younger brother Kenneth Mervyn McWilliam: here is a birth registration for 1908 with a Kenneth Mervyn McWilliam born to the same couple who i think is Doris's parents: Robert Morrison McWilliam and Rose Molloy:
https://search.ancestry.ca/cgi-bin/sse.dll?indiv=1&dbid=1778&h=2088168&ssrc=pt&tid=118252102&pid=100170314317&usePUB=true