Not sure how to solve this problem. For the most part, unless the mother is certifiably insane or in a prison for a felony, etc., the mother gets the kids.
The only thing I can contribute is if the father and in-laws state such a drastic change in upbringing does present a problem, and then hope the judge will agree so the children can at least be more, than less involved with mitzvos.
Hatzalacha to this family and hoping the mother comes to her senses are at least has rachmonus on her children.