Op, I think I understand how you feel. My DD "came out" in her teens, it was kind of a shock mostly because it the possibility had never occurred. You think you know a child, a sibling, then find out there's something fairly fundamental you were clueless about.
Take the time to readjust your thinking. In retrospect I felt I'd missed some clues - but she was a youngish teen and it was mixed up with the problems of trying to respect a teen's privacy, not pry but still be available - that whole impossible balance.
Also, it was over 10 years ago. Even though I had friends, her older siblings at uni had gay friends - times were different. I'd hope that nowadays we don't assume someone's sexuality, that there's more space for kids to
I understand your surprise, it doesn't mean you're intolerant or your gay friends are somehow fashion accessories not real friends! You probably knew they were gay when you met or soon after. You've known your brother all your life without knowing this, it might take some readjustment.
Especially as he's had to hide his real self due to your parents' intolerance. That's the real problem. I hope you will support him and be very firm and clear with your parents. You, too, may be forced to make a choice.
If you really think they will say something unforgiveable, perhaps you could spend xmas with brother and tell them why. Ultimately they need to accept their son - but it may be worth giving them more time to come round before risking a schism.
As someone said above, have you asked your brother what he wants? Most of all it's about him - he's facing the horrible possibility of severe rejection from his own parents. He's very vulnerable, I hope you can put him first.