Hi
on the other hand maybe John was Sydney's son, and Lillian was John's step mother. Maybe Lillian was Sydney's second wife as it seems strange no mention of John on Lillian's Memorial. Lots of maybes

May I suggest that you contact the S A State Library asking if they could send you a copy of the death notices, that may solve the mystery, and if you are lucky may also give info on Lillian's family.
Cass