Also, no matter the circumstances, I would be really, really irritated to be told of the fact that I have to bring a dish, with no chance to explain or compromise.
And OP heard that through the grapevine, right? No organizer called OP. It was a relative called mom and mom told OP. I can't stand gatekeepers. If you want me to bring something, give me plenty of notice and tell me directly. That way, if I have a problem, I can respond directly.
ETA: I went back and read the original post.
Dying great aunt's daughter, Cousin J, called Mom's sister, who called Mom. And Mom told OP (I don't know if OP lives with Mom or not).
I think it's like a game of "telephone." By the time the info got to Mom and OP, who knows if it was originally a request, a suggestion, an order, etc.
If I were OP, I'd either call Cousin J directly and clarify, or just feign ignorance/misunderstanding/uncertainty and show up with something (food or gift) that I had planned to bring originally before all of this game of telephone started.