17 year old prego sister and her controlling boyfriend.?

You can't do it for her. She has to decide for herself that she wants and decent respectable life. Until she has had enough and has the desire to change no one can make her. Sorry but this is just a sad fact of life. If you really don't want to go back and be involved the do not do it. It does after all put a lot of pressure and stress on you, but you alone have to decide if you can deal with it.