Given the circumstances and what she said about how her family thinks you're trying to scr3w her in the D, does she actually think they can now all control THE obvious tension in the air, just for your D8?
Good grief. And highly unlikely.
Even reading about it makes me anxious.
I'm still new to this so not sure how great my advice is BUT I would thank her very much for the invitation and agree that it is definitely all about your D8. Therefore, you want to make sure her party goes off without a hitch and since kids are very perceptive, even if everyone was on their best birthday party behaviour, that the underlying feelings of discomfort would still be there. And of course it would be in your D8's best interest to avoid all that stress.
Or make it simple and tell her you're just not ready for them all to be in the same room and that you will make sure D8 has a great time with your family at your celebration. What kid wouldn't want two parties? That's definitely making it all about her.
Good luck. These are the kind of situations that fill me with dread about D. Ugh.
And highly unlikely.