How far do you go to help somebody that doesn't take care of themselves?

I am 26, full time working RN, raising a 16 month old son and baby girl on the way with my fantastic husband who stays at home and takes care of house, family, and does mechanic side work for extra cash. My mother is 48 with history of back problems and mental illness. My dad is 47 and works full time.

Life growing up in the household was highly volatile to say the least, but after me and my younger sister moved out and started finding wonderful, sweet emotional and financial independence their behavior started spilling out of control. Among other behaviors, physically abusing each other enough to both receive assault charges and going to jail on multiple separate occasions, as well as almost getting into a physical altercation with my mother in my own home 6 weeks after giving birth, kind of sealed the deal for me as far as what I'm comfortable with in being around them and having a relationship. Basically I'm not talking to them until we all get some professional help.

Now as a nurse specializing in neurological and cancer care I've seen my fair share of disease. Enough to realize that my mother is NOT all the diagnoses and level of physical dysfunction she says she is. Not saying she doesn't have medical issues, but not everything she tells us, and the majority of her symptoms I can identify being attributed to lack of care. Strictly from the nursing perspective, which is not all knowing, and with limited knowledge of what's really in her charts.

My sister is also an RN and 22 years old, just starting out in her career. She has always been viewed by my parents as "the sweet one" and I've felt like a bit of the black sheep of the family. She definitely takes more crap than I do. But now she's spending all this time taking care of my mother when she should be focusing on herself and living her youth.

I think my dad should be the one taking responsibility for my mom if she's truly mentally ill, but he continues to work out of the house on long contracts and leaves her alone, just like always.

I grew up in the role of survivor and protector. Withstanding my mother and father's cruel comments about how terrible I am to prove them wrong in what I do today. Now I'm trying to focus on my own family and let go of people I can't help. But the feelings of guilt and sadness overwhelm me at times and distract me from what I need to do. I'm scared for my sister not living her own life and continuing to be twisted by my parents. I'm scared for our upcoming baby girl and if I have what it takes to be a good parent. I still love my parents and desperately want better for them. Anybody out there with words of wisdom or encouragement?