Who says you have to do this with your sister? If it's not a commitment in your best interests, or even in hers, and it doesn't sound that way, you do not have to keep a commitment like that.
And who says you have to be committed to self-sabotage either?
Self-sabotage is like a self-fulfilling prophecy, you end up doing/being what you think and what you think you'll do/be ... you think, oh well, that's what I do, I sabotage myself, so then actions follow thoughts/beliefs. And then you think, see, I sabotaged myself again; that's just what I do, guess that's the way I am/it is.
But why stay stuck there? Why not stay committed to a course of action, like therapy, that will help you learn how to stop making such self-fulfilling prophecies?
I think everyone makes mistakes and sabotages themselves, intentionally and/or unintentionally both. God only knows how many times I have done both! But it doesn't need to be allowed to be a pattern, a belief, a label you've given yourself .... and if it is pattern and a belief, why, it's a pattern and a belief that can be changed! Why sabotage that hope of change by not giving it a chance to happen in you?