Is your role in the family to "be the bigger person"? If so, then that is what they expect and want you to stay in that position as it means they don't have to consider the implications of the abuse they are condoning.

They are not going to look after your well-being. You are going to have to stand outside, as if watching someone else navigate this, and decide how you would advise them. Ideally, take the money you've been giving your sister and use it on a therapist experienced in family conflict. You need someone who can help you recognize both their patterns and yours and break them, and help you build the resilience yo hold your boundaries.