Bokura ni Namae wo Tsukerunara

...If only he hadn't found out they were brothers. In the spring of his senior year, Izumi Tosaka is reunited with his little brother, Kaoru Yano. The two were separated in their parents' divorce seven years earlier. Now, however, the two share a dorm room. Kaoru's voice has gotten deeper, his hair is now dyed red, he looks more grown up, and he has a totally different last name... So much about Kaoru has changed, Izumi doesn't even realize they're brothers at first and finds himself falling for Kaoru. Even though they're brothers, Izumi can't help this strange new feeling welling up inside of him...

(Source: Renta!)