Wow..that is horrible! If they really wanted to be at their son's wedding, they would be. How does your FI feel about it? If they don't want to come, fine, that'd be one thing. But to continuously harass you about it and try to make you feel guilty about it..that's just so wrong.
I'm so sorry you have to deal with that crap...this is supposed to be a happy and exciting time. Well, you'll still have a great day and you'll be enjoying it with those that really want to be there. Good luck hun