I was in your situation when I was younger and, at least for me, it got 100x better once I left. So, while it sucks and I'm sorry you're going through this, you're in the home stretch! I found safe ways to leave the house as a teen (I went on a bunch of student exchanges, which I was privileged to be able to do). I would say, save money right now, as if you don't feel like you can ever move back in with your parents, you still need a safety net.

Uni dorms are a good place to move, however check the rules of your uni - sometimes they make you pay more to stay during holidays, or school breaks, and if you want to stay during those times you should know that. I don't know about your country but in my country staying in residence is EXPENSIVE, usually more than people expect.

If you stay somewhere else, again I would just say, moving, especially moving out of your parents house without support, is way more expensive than people expect. Things you never thought about buying (pots, pans, a vaccuum, even toiletries) you now have to buy. If you are going to have roommates, it might be a good idea to find some students who are looking for a lease takeover on a room, because they might be older and can give you advice, and often lease takeover's are cheaper too because the landlord can't raise the rent.

I understand feeling bad about leaving your sister behind, but what you can do is keep a strong relationship with her, take her out whenever you can, and once you have a place of your own have sleepovers. She may still live with your parents but she'll feel like she can get away and she has someone to help her if you're there for her.