my sister is in the middle of a divorce, my brother in law is threatening her telling her he is picking up the kids and not giving them back, what rights does she have
BEFORE she gives him the kids she needs to get an attorney and file for custody.
I made the mistake of giving my ex our kids, he promised that he would have them back to me at a certain time, I go to get my kids and he had already filed for custody of our boys. It took me 2 weeks to get them back. The only way I did get them back was I tricked him. I used his feelings toward me against him, cried told him we could try to work it out, that we didn't need to rush into the divorce so that I could to get my boys. I had seen a lawyer and she told me that since I hadn't been served the papers yet that if I could get him to hand me the boys I could file for custody too. Beings that they were in my physical custody I would be the one to keep them until court. But if he didn't give them to me willingly I would have had to wait until a court date was set.
It has been a battle since.
Your sister needs to file for custody before she allows him a visit. If she gives him the kids he could do the same and she will be the one fighting to see her kids.