I'm sorry to hear of current situation between you and sis, you and parents. As for the nephew....he's just a nephew..not your son. Really you don't owe him anything except maybe distant civility at this time. (Do I sound brutal??)

One day hopefully of the 3, parents and sister, you will have better relationship. It might not happen for a few years yet.

My personal feeling is that your sister is probably feeling emotional pain, as a mother of a son who hasn't made the best of himself, but she hasn't recognized that in herself yet or at least, won't let you know.

Wishing you the right times to say right things you feel most strongly from your heart/head when you are alone with each your sister, mother and father.

It will happen one day with at least 1 of them.

Because it has happened to me...several times over.
No family is perfect..and the bigger the family, sometimes the more complex it can be.

But it's for your sister to tell you if she wishes to move beyond blaming you for-whatever-it-is-that-is-bugging-her-so-deeply.

Carry with your life apart and free now.