I agree with HRU. Just tell her that the receiving parent will pick up from now on if she doesn't want to drive at least part of the way.
She may call the cops, but if you don't have anything specific in the paper now, the most they will do is call you to intercede. You just tell them that you in no way have refused the visit and you are fine with her coming to get them. She just has refused to come and get them.
Who is going to give her the money to hire a lawyer to start a court fight anyway? And even if someone did, she is living with someone else right now and the kids have no where to sleep when they visit her. She could chance loosing overnight visits until she comes up with a place for them to sleep when they visit.
I wouldn't worry about it so much. Still think it sucks that you all moved away so far cuz we've been on the other end of that. But, it seems mom doesn't quite got her stuff together at this point. If and when she does, perhaps you can think about giving her more time during the school breaks.