Hi
You may already have this information but here goes
Estelle's birth from FreeBMD and a possible brother
Surname First name(s) Mother District Vol Page
Births Sep 1932
Yates Robert F C Williams Bristol 6a 107
Births Sep 1939
Yates Estelle W L Williams Bristol 6a 371
and a marriage for her possible parents from same site
Surname First name(s) Spouse District Vol Page
Marriages Jun 1932
Yates Robert F Williams Bristol 6a 315
There is also a record for her on the National Archives of Australia which you could request
http://recordsearch.naa.gov.au/NameSearch/Interface/ItemDeta...Edit: Am unable to find anything else but the following website might be of use to you in finding your sister
http://www.childmigrantstrust.com/Karin