wow how awful
I'd be gutted too and would have been very angry with my brother.
Wanting a child free wedding is one thing but a family free wedding?! He is choosing to not invite his own nieces and nephews?
I don't know, I had a child free wedding but all my direct family from 0 to 99yo were invited...
Anyway at the end of the day their wedding their choice but you can chose what will become of your relationship with your brother.
PS does the bride has any nieces and nephews?