This yaoi oneshot was a favorite of mine to read from a long time ago, but I don't remember its title.
I vaguely remember the synopsis:
A low self-esteem young man (brother #1) is suddenly asked by his confident brother (maybe twin brother?; brother #2) to keep his rich boyfriend (I believe he was an injured athlete) company on an (island, maybe? I remember there being lots of water..sea?). On the island together, the man (brother #1) and the boyfriend begin to feel things for each other, but stop themselves; but the boyfriend gets mad when he finds out that the man's brother (brother #2) doesn't want to be with him anymore (he is shallow and I think he wanted to leave him because of the boyfriend's injury) and believes that he (brother #1) and the brother (#2) are making a fool out of him. He has angry forceful sex with the young man out of revenge. However, in the end, the man and the "boyfriend" confess their love for each other and the man's brother is okay with it.

Something like that...it had very romantic scenes (especially this one water scene) and I want to read it again after so long, can you please help me?!

first story of Tsumi to Kuchizuke
https://www.mangaupdates.com/series.html?id=1722

That's it!
Thanks!