Hey bajaloverz, sorry to see you're feeling so guilty. The guilt trap is so easy to fall in especially when you list off all the awful family relationships she seems to have. Do you feel like it's in your power to "fix" her or make her happy? If so, why do you feel like it hasn't happened yet?
Have you read this article:
The Symptoms and Diagnosis of Borderline Personality Disorder? If you take a look at the beginning of the article and the beliefs that a borderline person has, a lot of them are contradictory. It's almost like they are programmed to "fail".
Seeing how your FiL is today, are you afraid that you'll end up being like him if you stay with your uBPDw?