Futanari Haha to Josou Shounen to Haha no Shasei Kanri o Suru Manamusume.

Miki, a mother who had sex with her own daughter, has become the object of her daughter's observations, using it as an excuse. The daughter's repeated sexual acts, which she calls "experiments," escalate. Then, one day, her daughter's friend (male) comes to visit. He casually peeks into Miki's room and sees a scene of Miki being raped by Miki. The boy is confused, but his crotch is abnormally hot...