Since the appointments are essential, have them scheduled on your wife's time or make plans assuming dad is not going to take her. It is easier to plan for the likely event that he will not take her than to have to arrange something at the last minute.Some people are self centered jerks. Don't risk your stepchild's life because she has a parent with other priorities.
Getting a change in the orders is secondary to getting her to her appointments. You all are so fortunate she is doing so well! The inconvenience is worth it.
The thing about beating your head against a wall is that it feels so good when you stop; I don;t know who said this first, but it is so true.