Given that there is a history of allowing visitation, I doubt a prosecutor would bother with such charges so long as the child is returned. It could happen but I'd put the odds less than 50-50.
Mom needs to talk to the ex about the potential plans. If she knows a trip out of state is occurring and doesn't wish for her other son to participate, she should not send the other son for that particular visit. Maybe discuss whatever reasons she has for not wanting the other son to go with the ex and see if any kind of compromise could be had.

