I mean, Chaos, nor its subfaction Chaos Undivided, have direct ties to transgender (apart from Slaneesh and its demons being a literal amalgamation of man and woman), but generally represents countercultures opposing today's conservatism. I'd say that those who enjoy Chaos factions, at least lore-wise, are those who can't deal with the conformism conservatism demands, especially since it at times goes against man's urges for self-exploration and finding one's place in this world.

I'm sorry your brother is turning like that. Factions like those who Trump belongs to use fear and anger to coerce others to do their bidding. If you can stomach it, maybe it might be good to have a little heart-to-heart with him? To see what actually scares him. However, don't beat yourself up over it if you can't handle that confrontation. Sometimes people like him are so deep in the delusions of these factions that it's similar to talking to a brick wall.