Anony,
I think it's essential that you get to set the boundaries for your own life...boundaries that keep the good in and keep the bad out.

The harder issue here might be giving yourself permission to not only stay away, but to stop giving them free rent in your head.

I can completely understand your not wanting to go, your questioning if it's 'wrong'....I went through that same thing Labor Day wkend of 2006. My extended 'blended' family had a big reunion in Estis Park...I chose not to go, and it wasn't even about the money, tho many chose to believe that it was because of the money to get there, and also chose to believe it was because we had just moved (both true, but not reasons why)

In my families case, a family reunion is just a poorly devised tactic for them to continue telling lies that everything is okay....

Also in my case, it was one of the rare occasions that my older brother who is a quadraplegic, traveled to the reunion....since he had abused me, they still couldn't understand why I didn't go. There was enormous pressure tactics put on me to try to 'get me' to go...my absense was loud and clear, it isn't okay, this family is not safe for me to be there.

Back to you ~~~ whatever you choose to do is perfectly okay. Take inventory of what would fill you and bless you and honor the goodness in you, and it's perfectly okay for you and your mom to not go. You might even schedule that time to pamper yourself and your mom to distract that lying guilt that says you're doing something wrong....you're not.