WOW - this is one story for the books - I am sorry that you are going through this.
I agree - you weren't part of the "Planning Committee" so you are not required or expected by any means to contribute to the party. If you had been and had discussed it - then perhaps yes - but in this case . . . NO!!!
Sounds to me that you are not that close with your Aunt and Uncle - they should understand - at least I would hope!!
I would personally send a note directly to your Aunt and Uncle explaining your sadness for not being able to attend and leave it at that. You could explain to them that you were unaware of any of the planning for this party - you never know that your cousin has said to them (not that it really matters - but to at least give your side of the story). They may not even know - that is my guess!!
good luck and many hugs!!!