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I'm looking for a manga I've read some time ago, the main story was about two high schoolers, and there was a side story about the younger brother of one of them. The little brother was starting to develop feelings for his friend (both were like 12), and when his brother found out he got angry because he though his brother was too young for this. Then in the end the little brother goes to the friends house, confess and have sex

Could you please help me?

Is it Tsuyogari?

It is! Thank you very much!! 😀
If you could help me, that is another one i want to find, the begining was like two boys living in a place like an island or something, one of them had a perfect dark hair and is very disciplined kid, I think he was an artist too, and they went to school with that anime-style perfect uniform. I've only read the beginning, but they they were like 12 at that point. I know is not Bara to Yaban, I've already looked for it.
Sorry for asking too much, and thank you a lot <3

I'm really sorry, I don't know about this manga. I tried searching but couldn't find it.
These are two I found similar to your story
Ameiro no toge and Star-like words.
I'm really sorry I couldn't help ;_;

No problem, thank you very much <3