Visit
Frequent Flyer Terms and Conditions | Qantas and do a search for "Eligible Family Member". It currently says:
'Eligible Family Member' of a Member means any person the Member can demonstrate, to the reasonable satisfaction of Qantas, as being related to that Member in any of the ways described below:
- Husband/Wife
- Parent/Step-parent
- Domestic Partner/De Facto
- Child, including foster and step-child
- Brother/Sister
- Half Brother/Sister
- Grandparent
- Grandchild
- Son/Daughter-in-law
- Brother/Sister-in-law
- Father/Mother-in-law
- Uncle/Aunt
- Nephew/Niece
- First cousin
I don't see any more details about proving the relationship, but I haven't trawled through the details. If it's anywhere, it should in this document.