Seisai Ryoku no Takai 7-tsu Shita no Imouto to Kisei Jijitsu wo Tsukuru Hon

A pair of siblings are so friendly with one another it's as if they are a newlywed couple. It just so happens that the older brother has been fapping off to Yui for quite some time now. However, one day, due to a certain turn of events, the siblings find out that their love for one another is requited and...!? (Source: DLsite, official)