There's a real difference in semantics or wording here as well that can help (sorry, sometimes I get ultra technical). You said that you don't want to abandon your sister, but that's not true. She's abandoning herself. You cannot allow her to suck you into her self-destruction. In the end, you're really hurting her more than you're helping her.

it's known as enabling. Every time you give her money, it destroys her by allowing her to not find ways of supporting herself. Every day you allow her to live rent free it's killing her because she isn't learning how to take care of herself. Instead of allowing her learning moments to teach independence, when you help her out it tells her that it's acceptable for her to be dependent on you.

That's all fine if you want her to never be independent. But if you really want her to have her own life, you have to set up boundaries so she can't lean on yours instead. If you teach yourself to see how your "helping" her is really killing her, it helps in gaining motivation to set those boundaries.

I hope this helps, although I'm sorry if it was too blunt.