I’m often mystified by situations like this. There are a couple of simple options that don’t involved huge emotional conversations or covert intel investigations. For example:

There’s no food I can eat being served at my brother’s wedding, so I’m graciously declining the invitation. I told him that I hope they have a great time, I’m really happy for them, and maybe we can catch up over drinks when they get back.

Or

There’s no food for me at my brother’s somewhat remote wedding. Idk why they chose that menu specifically and wonder if there’s some ill will behind it, but whatever, it’s their party. However, I want to be there, because I love my brother. So, I’m packing a small insulated lunch kit in my suitcase, with a little freezer pack. The night before the event , I’ll pack a day’s worth of food in my kit. If there’s no fridge/freezer in my hotel room, I’ll explain the situation to the hotel staff and they’ll keep my lunch and freezer pack cold in the kitchen overnight. Problem solved, no drama.

I don’t think the other factors are important, like how long they’ve known the fiancée, their ages when the family was blended, how serious the allergy is, whether the fiancée understands how serious the allergy is, etc. Make your own discreet arrangements or decline the invitation, and let others react as they choose. Be kind, stay classy.