I'm not going to let my fiance's parents have a say in the guest list because they are not financially contributing to our wedding
How does them not contributing make you feel?
I'm a little bitter, but I can get easily frustrated.
You shouldn't let financial contributions dictate how you treat people. It is petty to let money com between you and someone else.
It's not petty to me.
Did you even ask why hey didn't contribute? May be they can afford to. Why not ask them to contribute in a way that doesn't involve money, like helping send o invitations and helping set up the wedding area?