Late but for me it has more to do with the fact that since the bro is hetero, theres little to no chance of him bending for the brother so whats there to be mad it? I mean yaoi likes to make the straight guy turned gay trope so easy but in reality it doesnt go down like that. The gay bro is basically harmless, not a threat, cause the sexuality is a huge problem like it or not. Also theyre brothers, another obstacle. I cant be mad at a rival that basically stands no chance. Similiarly, if it was two sisters, cant be mad either for the exact same reason. Most ppl hate on girl rivals cause there IS a chance dude could fall or have relations with her. At least thats how it is for me.
I already knew what the comments were going to be like before even checking. Honestly no one gives a crap about characters. Only gender roles. Most are sexist and dont want to admit it. Even if it was the other way around, if the brother was actually a sister you'd all be giving them hell for it but its okay because its a guy. Then you wonder why people hate on females so much but you yourself are the ones hating on females so really, its not a mystery.