Haha Sannin to Ana Asobi

The protagonist, Ichihashi Souta, has a forbidden relationship with biological mother, Kisaragi Yui, and the stepmother, Ichihashi Akiho and indulges in a life of carnal desire with them. One day, his career woman aunt, Kisaragi Ai, finds out about their taboo relationship. Having learned about Souta and Yui's relationship, Ai decides to become a substitute mother for Yui herself, to accept Souta's sexual desires and to put an end to this inappropriate relationship.