I'm going to make a guess and say that your mom's response of "I'll have to ask your dad" was a way of stalling you so they could talk about it. I'll bet she wasn't really asking your dad's permission. That's usually the case when someone says "I'll have to ask ___."
I have food allergies, and I have to say that going to an event that is centered around eating is NOT FUN when you can't eat what's on the menu. I understand the suggestion of "eat a big meal ahead of time and then just eat what you can" but I have to say that anyone who suggests that has never had to sit at a table and nibble on 2-3 side dishes while everyone else enjoys a full meal!
I would say that eating a big meal ahead of time would work in a situation where you weren't very close to the host, or the host had no way to know or accommodate your dietary needs.
Since it's family, I think bringing your own entree is fine. I have learned to do it myself and no one has ever said anything or made me feel odd for doing it. Most hosts WANT their guests to have a good time and enjoy themselves. Enjoying the meal is just part of being a good host.
If your fish doesn't have a smell then I would think it would be okay but I can imagine the smell of fish competing with the smell of turkey might be gross.
Good luck with your meal! Happy Thanksgiving!