@Poppymonty3 YABU but understandably you are feeling the pressure of the wedding, after three years in the planning & waiting all through Covid, looming with so much to do & someone switching their plans will feel like a knock. But as much as this day is important to you, there is time still & the vagaries of life to come so there is a chance that others may drop out as well. Please try not to take the disappointment so personally as you run the risk of cutting off your nose to spite your face. So far you have laid into your brother about something beyond his control & now you have lost his goodwill. Scrap the printed seating plan as definitive, there is always adjustments for cancellation or unexpected guests.
Yes, your nephew is special to you.
But he is 15 yo & you are just his aunt.
In a couple of weeks it is UK half-term for most schools, reasonably his mum has booked somewhere for a holiday for her family. He is just 15, he wants to go on holiday with his mum somewhere that he knows & likes.
After Covid lockdown, disruption of school & friendships, the existential threat of war/economic meltdown & the slings & arrows of outrageous adolescence, the kid would rather go on a familiar holiday with his family than spend a day togged up at your wedding with his dad where he doesn’t know anyone & there is nothing for him to do.
To have expected him to be an Usher & undertake responsibility of turning up on the day to perform formal duties is too much. I would not ask this of my 15yo nephew as it’s enough for him to manage a school uniform, travel a few miles & get his homework done on time - he has to apply himself to this otherwise he’d rather be off all day doing what whatever floats his boat.
If he were an adult, I would expect him to take the responsibility of letting you know himself rather than having his dad do it. But he is not, so Dad did that for him.
Do yourself a favour, take a big deep breath & make up with your brother.
He may still make it to your wedding.