What do you do in certain situations?

As most of you well know, my kid sister is getting a divorce. However, she says her husband has changed dramatically over the past year and a half. I was shocked when he told that at first. Then I realized maybe it was not him that had changed but her. I'll try not to degrade her but the facts show that she has multiple affairs with multiple partners. WHich sickens me. How can you say you love someone and marry them and then do the most stupid and idiotic thing anyone has ever heard of. I guess the worst part of it is that she wants to take daughter and basically run from her family because she knows we do don't condone the decision she has made. Not only does she want to take her daughter and leave but she wants to keep he rdaughter around people who do drugs and that Zoey may be able to reach the drugs and intake the drugs. My parents are not going to let her mother take her down with her so they have stepped in as the savior and for now are keeping Zoey. That is until her mother can figure out how to get her own daughter back. Our family is just a mess. Just when you think everything is ok, things are not. I mean what exactly do you do if the mother wants to harm the child. That is what it all comes down to. Right now I guess our family could use a miracle on mommy and babys behalf. Thanks for reading my thoughts.