Yaoi manga which the plot is about a guy stranded on an island, who meets his brother for the first time...?

So I found this manga a long time ago and i loved it. Dx It's about this guy who is stranded on an island and he meets these people there and amongst them is a boy who apparently is his brother... >.< And then there is something about mermaids being the ancestors of the people who live there or something. I don't know. But the guy who washed up on shore is the seme and the guy's brother is the uke. >.<" Help me please?