It depends on what level you refer to. By that I mean; are you physically care taking them, or emotionally?
I suppose you mean emotionally, as would I. My mom suffers from bi-polar and I have been the parent of that relationship since I was 16. At first I resented the role, because I was so young and hadn't squared with the whole situation.
Now that I have I feel I am in a better place of understanding. The first thing I would recommend to you is that you stop internalizing anything they tell you as coming from a parent, but rather a person with emotional needs.
If you can do that then it will be easier to deal with the stress the relationship causes because you won't take any of it personally. From that point it becomes easier to manage.