I'm going to mess up the ground rules with my MIL
How are you going to do that?
I can't tell you and I'm not entirely sure what I'm going to do.
I am not sure what you trying to accomplish by doing so. It will bring more pain than gain.
I want them to feel bad for what they did.
Well, now it sounds like retaliation. Look, if you break the established rules, you're just going to make the situation worse.