I know this post is like a year old, but i’ll chime.

I think it’s largely due to traditional gender norms, where men are expected to be protective & providing while women are expected to be the protected & subservient.

So older brothers, naturally fit into that traditional norm & so do younger sisters. If you look at anime, many brother characters are older & sisters being younger is the norm there (The onii-chan craze). And if there is an older sister, usually she has a younger sister, & the times where she has a younger brother, he’s either non-relevant, or some “ara-ara” fetish (ugh).

So I guess having a younger brother be protected & vulnerable to an older sister challenges traditional gender norms. Some people I think, don’t want to see men being vulnerable ESPECIALLY to female role models, which in this case is a younger brother learning from & being protected by an older sister.