I'll preface this by saying that I am not religious. At all. However, my opinion about asking "What would Jesus do?" is this: It's irrelevant. Jesus was God himself, no? Well, okay the Son, but basically God. So He has resources that ordinary mortals don't. The more relevant question, to me, is "What would Jesus say that I should do?" Knowing what is in my heart and mind and what I have available to give, what would He say I should give? How much would He want what I give to cost me? Do with this whatever you wish, but that is my take on it.
Apart from that, I do not believe you are at all obligated to sacrifice your own mental, physical, psychological or financial wellbeing for anyone else. If you truly WANT to do that, because you care that much for the person, then you can, but to do it out of guilt, because you don't want to have to deal with the negative emotions or beliefs that would result (I'm a bad daughter, I can't enjoy my life knowing the condition she is in etc) is selfish, because you are only helping her to make yourself feel better, not to actually improve her life. And in spite of that, you WON'T feel better, because how could you, by making your own life worse?
You were not responsible for creating any of this mess and you are not responsible for fixing it. Even if she has a mental disorder (of course she does). If she knows on some level this is not normal behaviour, she had many years in which to get help for herself. If she is so impaired that she doesn't know her behaviour is not normal, how exactly are you going to fix this? Are you a psychiatrist? Helping her psychologically is above your paygrade. If she had cancer, would you try to treat her yourself? No, you would try to take her to a doctor, and if she refused to go, there's nothing you could do about it. This is no different.
There's nothing wrong with making yourself your priority here. In fact, you need to, because not doing so will help neither yourself nor her. If you need counselling to help you get through the emotions and beliefs that will get you to this point, then by all means get it. I would suggest that you go to a non-religious counsellor first, to get their viewpoint on it. Follow up with a Christian counsellor afterwards. Then you will have have two perspectives from which to make a plan.
Best wishes. I hope you can find some peace for yourself in a difficult situation.