You started out saying your father in law wants you to indulge this low life woman to be good Christians. Kind of a joke there. are there any bad Christians! I am really tired of guilt trips being laid in the name of religion! Please this woman has nothing coming from anyone! Especially you!
I do not understand the control she has over your father in law, but one word comes to mind and that would be whimp.
I don't care if he buys all your food or not. that is your home and his ex is a guest and an unwanted one at that. You totally have the right to say who is allowed in your home when you are not there. He needs to understand that and have some respect. Actually it does not sound like anyone has respect for you or your husband. That is ridiculous.
I would not do a thing for that party and I would not allow anyone to make you feel guilty. No one deserves to be disrespected in their own home. You and your hubby need to make a united front about that! And don't let them walk on you!
Sometimes it does not pay to be so nice, especially when you get walked on because of it!
Sounds like you have been way to nice for way to long. Good Luck