Menage a Twins is a bit of an Oedipal story, just without the dad.
A young man named Minoru meets Akane on the Internet. It seems she tracked him down, then arranged for their meeting and seduced him when she got him alone. Minoru instantly becomes the big stud in the apartment building, and lonely wives take note. But there's a twist.
Turns out that Akane is the older sister of Minoru's mom. And as the truth is further revealed, it becomes apparent that Minoru's mom isn't at all who he thinks she is.
Retailing: Incestuous anime.