Urgent, PMV TO 801. ADDITIONAL APPLICANT ENQ! HELP!

Hello,

My partner has recently arrived in Australia on a PMV and we have just gotten married.

The PMV process took under three years after being declined, remitted to AAT and then finally approved. In this time, my partner's mother has fallen ill with cancer.

Now that my partner is here, we are married we are gearing up to apply for the 801 partner visa, my query is can my partner add an additional applicant (his 12 year old sister) to the application even though she is back in Ethiopia, as his mother has taken a turn for the worse and doesn't have long or do we have to apply for him directly first, wait till he receives permanent residency and then apply for the orphan relative visa for his sister.

Any tips, advice would be really appreciated.

Thanks in advance