Find a lawyer. School normally has legal

consulors. Maybe free for initial consultation. Don't worry about spending money on lawyer now.
If you are in CA or other community property states, even your LG pay mortgage and etc., you are at least entitled to 50% all of your assets (including the money in your LG's bank account, his 401K). In your case, you probably will have more than 50% as you are going to support your children, even for non-community states.
Don't worry too much about custody fight, it does not seem your LG is willing to take care of your children; especially if your in-laws have no interests taking care of the kids either. However, for the sake of your children, you should not try to block his visiting (or partial custody) right.