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.