I wouldnt offer to help at the next family gathering.
If you do offer to help - ask for a very specific task "Do you need someone to fill the water glasses?" (now that you know how to do it properly, of course,
) and when you are done with that, walk away. If she tries to say "oh, could you..." I dont think it would be rude at all to say "Oh, I am just going to catch up with Uncle Jimmy, I'll send someone in to help." Then go into the living room, and state "Auntie Disorganized needs some help in the kitchen." and go about yoru business.
Alternatively, just say "Ok, I'll do the water glasses and put the silverware on the table. Doing that will still give me time to catch up with Uncle Jimmy." And then walk away.
You could add, if she persists "Oh, I hope you understand, I was planning on catching up with Uncle Jimmy a bit before dinner."