Even when they're with her, all they're doing is watching movies and eating pizza or McDonalds. Even in her own place, she has no idea how to make dinner.
Every time the kids stay with her, ask your oldest girl to tell you exactly what their mom has been feeding them and keep a written journal of it. Your lawyer can use it to cite that she is not providing nutritious meals for her children the way a responsible parent should. This would be a good ace-in-the-hole for your lawyer to use in the custody mediations.
You should keep your eye out for any opportunity to discredit this woman. No playing nice with stakes this high!