Its hard, hey.

MIL is having alot of trouble with my 18yo SIL. She has finally, after 2 years sitting at home, decided to go back to tafe.
I don't see what it'll do for her TBH. I believe she's only doing it to get the IL's off her back, because it was that, or get out of their house.

She also went through a stage of stealing & using her older brothers compassion to get money out of him when it suited her.

Have you talked to your DS about this at all? Maybe make an agreement that you'll help find accomodation for him in Melbourne, but he needs to have the money to support himself in the bank first. IYKWIM. At least a month or 2 worth of rent/food money etc?