There really isn't any advice you can give your sister that she doesn't already know. She already knows that she's married to a Peter Pan with hormones. Unless she's completely naive, she knows what she married and knows what she needs to do, but for whatever reasons, she's not willing.
Your sister's situation sounds exactly like my sister and her common law husband. She's been complaining about him for almost 20 years. I used to offer advice but finally realized she wasn't going to do anything about it but complain. I just had to learn to stay out of it. When she brings up something that he's doing now, I listen for a few minutes without comment, then change the subject. She's pretty stubborn sometimes. It takes a few tries to change the subject.