Might not be this, good read anyways
http://www.mangago.me/read-manga/nights_before_night/
I hate to say it, but it not the manga I was thinking of. But I have read this one and its a really good manga. But the one I have stuck in my head is more on the substitute side of things. Like I think the twin blames himself for his brother's death, that he forces this relationship upon his late brother's boyfriend. (I believe).
Its unfortunately not, but I've read it, and yeah its pretty good.
Could it be this one maybe? The dead one was a girl but somehow it fits? http://www.mangago.me/read-manga/kimi_no_naka_ni/
I'm trying to remember a Yaoi manga that revolves around a man who replaces their dead brother (could be twin) for healing his late brother's boyfriend, Who I believe he was already in love with. But I believe they fall in the with each other in the end. I barely remember the manga.