I don't know what to do.

Long story short. 
My sisters ex boyfriend from 16 years ago found her on facebook. He was in prison for 5 years. Got out two days after Xmas. He was pouring his feelings out to her and she did the same thing a day or two ago and even said she loves him. She's still married. Wants to get a divorce (because of her ex). Because he says sweet things to her etc. I've been telling her for years she should get divorced because her hubby is an asshole to her. She doesn't want me to tell anyone because if they find out her ex is a felon and their in contact she could lose her kids. This is the same ex my parents hate with a passion or they did 16 years ago. idk what to do. She obviously knows it's wrong if she wants it to be a huge secret.  but why all of a sudden does she finally want to go through with divorcing her husband? after he says things to her like: I'll love your "babies" (she has 4 kids). And I've always loved you and so on. she tells me he's been saved and he's a Godly man now but if he is why keep him a secret or at least keep the felon part a secret. My parents deserve to know and me being in the middle of it all idk what to do. Let my parents know what's going on or just leave it? I'm so lost.