Wow! Sounds dangerous. My sister was with a man who did the very same thing, brainwashed her into believing whatever he told her.
If you have tried to hear something about the kids and she won't contact you, I would either get the police, or her brother to go with you and get them.
Don't you think it's time to get a restraining order to keep OM away from your children?
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!