"Hi, " (so and so)...thank you for the invitation for your party. I just want to let you know that I'm sorry we are not able to attend but....I hope we can get together with the kids soon".
Why can't you come?
"It is just not going to work out for my family and I for Sunday night. Hopefully another day will be better. I hope you have a great party. "
If she persists, after you have said NO a third time would be just plain rude. If you have allowed her to grill you every time....don't allow it this time. Family time is just as important if you just decide to sit around, together, in pajamas all night. You are doing something important...you just have decided NOT to do it with her. Good luck.