Never been in your situation but I think I would ask myself this,
Is supporting your sister affecting your health and well being or your families? if no, then continue to support her as she is your blood, your sister.
If yes, well you and your family comes first.
But I see no reason why you shouldn't explain to your sister that you cannot continue to support her if you are being affected by it, that you may need a break from the drama. On the other hand, I would be devastated if my mother suddenly withdrew the listening ear, the being on my side, the support she gives me so I can feel for how your sister might feel if she doesn't have your support. So I guess I am saying "gird your loins" and take on round 2 with your sister! Good luck