It really depends on the kind of relationship you have with your father, and how much you want him involved in your life going forward. Him paying for the wedding does complicate things, but at the end of the day it's your wedding, and your parents should be there to support you, all of them.You could possibly have her included in some other way, giving her a corsage as well as your mothers, or something to that effect, and having her seated in the front row with immediate family, but if you don't want her walking down the isle that's your choice and you should sick to your guns. And as harsh as it might sound, your parent's interactions at your wedding are not your responsibility. They're both adults who love you and should be able to behave civilly on a day so important to you. Maybe have a conversation with them (desperately) before the day, telling them that you expect them to behave appropriately.