Birth certificates for twins have now been viewed on line. Parents recorded as William Robert Craig & Elizabeth Craig (nee) White. "Supposedly" used in original query because the only evidence to date of parents names is from FamilySearch.org, re Marguerite Cecilia Craig's marriage to James Edward Fitton in Toronto, Canada. If family story is correct that Marguerite Cecilia was sent alone to Canada at an early age she may not have known who her parents were?
Transcription agent has provided me with a Death Transcription from NSW (Australia) Registry of Births, Deaths and Marriages for Marguerite Celilia. Informant was eldest son. Name shown as Margaret Cecilia Fitton. Mother and father recorded as unknown. Place of birth shown as Ireland. I believe this record demonstrates a lack of knowledge of detail by son. Family Story is that Marguerite Cecilia was born in Belfast, but no proven evidence nor shipping record from Ireland to Canada.