I can't post anonymous here, so I'll just say that I can relate to your situation a lot.
I'm not sure if this is the answer you're expecting, but I think for now, while you're living under her roof, you're just going to have to put up with it. I also think you're going to have to cut your losses with that $30k. I don't think you'll see a cent of it.
As for the long-term though, the best thing you can do for her and for yourself is to cut her off financially. She will hate you for it, but you love her and it's the best thing you can do. It's a harsh analogy, but she's like an addict; you have to help your mum help herself. You'll need to be tough enough to withstand the emotional blackmail that will follow. Your car should be in your name, and you should have your own bank account. If you have a job, never discuss how many hours you're working, how much money you're earning, or how much money you've saved. Don't give her any more money.
I'm not sure how your step-dad plays into this, but you'll need to be ready to catch the pieces in regards to your siblings. You may need to support your siblings, but you should find ways of doing so that do not equate to you giving your mum money. Personally, with me and my siblings, I used to give them lunch money (I told them they had to spend it all or else give me the change), take them out for dinner heaps, and take them clothes shopping whenever I noticed they was outgrowing their current wardrobes.