You are right... the big brothers could give the little bros more support and talked them about sutuff like "it's not bad to be gay, there is nothing wrong with it", "we are in love which means that..blabla"... "we're sorry we were so careless you saw us doin it"... "do you need advise about something??"... But they seem to only care about themselves. jerks
Yes, the younger brothers were only in middle school and I think the problem was not that the older brothers were having gay relationship but that they let them see the sex.
It may have been better if the little bro already had it all figured out. I remember how I myself felt that sex itself is 'dirty' in my puberty and took some time to grow out of it.
I can imagine how hard it must have been to have to go through so much at once, like all at once he had to realize what gay sex is like and that he is interested in his childhood friend, and that making it harder to grow out of the 'sex is dirty' trauma.
I don't think it was only out of cowardice that the boy couldn't accept his first love.
He took three whole years to have sex with a girl he was dating, which I think kind of shows that he was slow in sexual growth.
I feel really sorry for him.
I didn't like how the older brothers handled the situation. Yes, they had nothing to be ashamed of, but it felt kinda... Idk... Regretful(?) that they didn't try to explain anything to Shiro? They just kinda left him to be confused and thought about themselves. Again, it's not that I blame them, I just value family relationships (a heart to heart would've been really nice imo)