Hi A.,
Is there a court order for her to go to your ex's family? I assume that there is one to keep the father away, obviously he is not following the visitation arrangement.
I suggestyou get a lawyer ASAP. Take the ex's family back to court to challege the visitation arrangement. Probably difficult financially, but necessary.
They are not providing a nuturing enviornment for her while in their care. Teaching her bad habits (poor hygiene, language and lying). Confusing her and distorting right (which you instill) from wrong. Who know what else she's hearing & seeing that she hasn't brought home yet.
Secondly, they are irresponsible in allowing the father to be there when she's there if he's not supposed to see her. I would think that those concerns would be supported in court. Supervised visits may be the next step.
She is at an age that makes her so vulnerable and easily persuaded, which could lead to problems for her later. They may be using her to hurt you. This unfortunately happens.
I hope that things work out for you and your daughter.
Good Luck!