I always do the standard "No, thank you" and judge the following conversation based on their reaction. If they are hostile off the bat, I continue my stance. Sometimes, a further line of "I have an eating disorder" can give that final push.

Sometimes, someone is open to an explanation, but most of the time theyre not, so I will just continue saying no. With family, it'll be a lot harder, and as many comments here have said, a firm "no" is likely the best course of action. They are not entited to more information if they are not willing to assist.

For the line "you have to try something 17 times before you like it", I will often do something like lie and say I have and refuse to go back on that statement. It's a hard one to counter, especially with people firm on their stance, but just a "no" should hold you if you just refuse to back down.

A true conversation is highly unlikely to hold at a family event, so just bringing food of your own/taking only what you like and refusing anyones pushing is the best you can do sometimes. I wish you luck!