Tell GD that you have to follow the court order and so they did.
Court is The Real Authority. By saying, "We have to follow the court order" it diverts the attention away from yourselves. In other words, it's not you saying No, it's the court saying No. Of course pwBPD will still play the bully, blaming and guilting games but pointing to the order works with police and others in authority and should at least weaken the ex's and ex's advocates' demands
In addition, D's ex's wife probably has no legal standing in the court order, she is not one of the parents. She could demand that the moon is made of cheese and it wouldn't make a bit of difference to the order.
Of course, GD15 is getting close to the age she will be able to drive and may 'vote with her feet' if pressured.