Aitsu to Ore

Takeru’s mother is about to remarry. That is a great opportunity for Takeru to live with a real family and, most of all, to have a big brother, Shin. Things do not actually turn out the way he expected, though.

One day, while waiting for his girlfriend, Takeru hears a couple of people discussing about money for sexual favours. Curious, he turns around to check out the chick, but it turns that both parties are male. However, what shocks Takeru more is when he realizes that the one prostituting himself is Shin, his new stepbrother…

First volume extra : Shounen S.

There's also a book of various doujinshi called Renai Junkie. The fifth story is a spinoff of Aitsu to Ore called Newly-Wed