All such options would be covered by Defacto and domestic partner. where not already covered by other options. For example a sister who identifies as male can be classified as a brother.
The OP could've selected defacto, that'd be the safe option. Pretty hard for Qantas (as opposed to the cops or ATO) to prove otherwise. Domestic partner, yep they come and clean my house every week, helping to maintain our domestic partnership...
Interesting to note cousin is not an option, but is now first cousin. So my cousins children can't get a flight from me. Not sure why mother in law is included - is there anyone who'd buy the mother in law a flight?