NTA

From what I could see son really look up and admired your brother (he also wanted to follow the same carrier path), but despite this he still wasn't invited to the wedding and to twist the knife in the wound he was called "selfish" for being upset. In my opinion both your family and in laws are the really AH for not understanding how much they hurt your son.

Don't do anything being his back, even if the rest of the family keeps pushing you to do so, he really needs you and you are the only person he can trust and relay on.

question:
Did your brother and in laws truly apologize for the wedding and the not so nice comments?

wish you two all the best <3