Hi!) I can't remember the name of the manga, please help)
The protagonist (P), his brother (B) and the brother's lover (L) are living together. P is in love with L. Then B dies in an accident (maybe arranged by L's evil brother, who was looking for the runaway couple). After that L disappears for some time, and then P finds him on the shore, or something like that...
I think that there was some locket that L wanted to have, and P bought one, but B already bought one for him...
Sorry, that's all I remember)
Thanks a lot in advance!

It might be Mata Kite Shikaku from So Cute?

I checked it out, but unfortunately it's not that story! Thank you anyway!)
UPD: Found it myself! Tsuki no Hikari mo Todokudarou

Ah, yeah, I read that chapter ages ago so my memory was pretty shaky 😳 Glad you found it, though!