This should be helpful in working back into the early years for Sybil before her marriage. I am not actually certain that the surnames of the witnesses were DOWNES or CRASE. It could be Downey or Crare. My knowledge of South Australian families is sparse. I also wonder if I have the clergyman's surname correct, but I doubt it would be significant to this quest.
So here is my transcription of the church record of the 1923 marriage, the image is readily available, no charge at family search.
Diocese of Adelaide
Marriage solemnized in the Chapel of the Collegiate School of St Peter, South Australia
Rex Leland BENNETT, age 27, born Snow Town, South Australia, Bachelor, Army Officer, residing Prospect, father as Thomas Charles BENNETT
Sybil Edith CRASE, age 22, born Sydney NSW, Spinster, residing Walkerville, father as John Robert CRASE
Were married in this church by banns this 27th day of October, 1923 by me, Wm H Irwin
In the presence of H Downes of Norwood (and) E Crase of Medindie.
Bride and Groom both signed RL BENNETT and Sybil Edith CRASE.
Film @ 007720194, Image 661 of 733.
https://www.familysearch.org/ark:/61903/3:1:3Q9M-C9BX-X1NF?i=660JM