First thing I would like to say is that no child should not be put in the middle of anything. I find it hard to believe (although maybe their judge was odd) that the any decisions having to do with Custody would be left up to the child. You didn't state what the childs ages were at the time of the divorce just what they are now.
I have been in and out of court for the past 13yrs and the one thing the court always says is "Do not pressure the child to make a decision or to have to make the choice over what parent you want to be with". NEVER put the child in the middle of a situation. The adults need to work it out. and leave the child out of it.
As far as the holiday schedule, both of my sisters are divorced and they split the Holiday's with their ex's, just as your husband does (1/2 with Mom & 1/2 with Dad). My daughter's father and I were ordered to share Holiday's every other year, alternating Holiday's. However, a few years ago he wanted Thanksgiving added and when they did, they didn't add it in the rotation correctly so now who ever has her for Christmas Day, also has her on Thanksgiving.
I know exactly how it feels not to be able to have a child with you for Holiday's and special occasions, however I have adjusted my life accordingly. Yes my daughter has to miss out on things, but that is part of living at 2 different households.
The only thing your husband can do is to file for a Motion through the court to ask for a modification of Parenting Time. He will have to show how the change will "best benefit the children" (not your husband). My daughter's father just recently brought me to court asking for everyother week in the summer. My daughter does not want this to happen, we ended up having a Guardin Ad Litem assigned to our case. This person is my daughters voice to the court system, although I thought he was very rude when taking to her and asking how she feels about things.
I would also like to mention, that you may want to take a step back and let your husband deal with this. I know my daughter's Step Mom runs things at their house and her father doesn't do much of the talking. This is only hurting the relationship between my daughter and her father.
As far as the 4th of July, just mention to the girls that you are going out of town and would like them to come if they would like to. If not, that is fine also, but you wanted to extend the opportunity for them to see extended family members that they don't normally get to see. Do not put any pressuse on them, if they say they want to stay home, let them, don't make them feel bad for making the decision....
Sorry if any of what I wrote may have upset you. I just wanted to give my opinion...I was never married to my daughter's father and have paid out over $15,000 in the past 13yrs for silly little things that he has requested and brought me back to court over and nothing ever benefited our daughter....