What do i do about my cousin and her kids?

My cousin is living with and marring a horrible man. She has 2 kids from a previous marriage. Everyone around her thinks that she can do no wrong and this man is a saint. What they don't know or are too much in denial to see, is that he is a huge freak. He has a wrap sheet that won't end. Been to jail from everything to ault, weapons, forgery, theft, and has been sent to prison and from there court ordered to go to the state mental house. Before he was sent to prison he was messing with a friends little girl. As soon as he got out he met my cousin, who is a cop and lives in a small town, and now lives with her and are getting married in June. She leaves her kids alone with them all of the time, and is blind to what he really is. She does seem to have this thing with marrying men who are sick. This is her 3rd. I don't care what she does with her life. But I’m worried about her kids. How do I go about telling her family? They think he's a saint and she wouldn't marry ANOTHER creep.