Thank you cupoflife - are brothers and sisters usually mentioned on death certificates?
I will check all this things on Trove. With regards to Mary Christina Brett - My information from living family Brett members is that no one knows where she is or any info about her at all. The only eye-witness story is that she did visit a brother in the 1930s but was not well received and also might have left under difficult circumstances (he thinks something went missing in the home - this is only a story from one of her nephews who was young at that time - that is all he knows apart from her being a family 'outcast' for reasons not known.)
Andrew Evans Thompson, her half-brother - family story is that he was an itinerant labourer and tended to live in 'hostels' and 'hotels' _ I did think that I saw Mary Thompson or Dennan once in the same place as him on something, but it was in the early days of my research and I did not know too much about the family then- I have not been able to track this down again, so maybe I made it up.
With regards to Juanita Christina - apart from the 3 sightings 1930 - 1934 on electoral rolls, she does not seem to exist anywhere else.
I will look for all the things you have said and would appreciate anyone else's searching too as in the past, I have come up with O in Trove searches, etc.