Based on the erotic game by Bishop. 
 
Itsuki's father's remarriage creates a family of three people who are not related by blood. 
Although the relationship is amiable, the new family members are beautiful women, and Itsuki is troubled by the closeness of his family. He sees his new relatives as women, and his lust grows everyday. 
 
One day, he caught his father cheating on his step-mother. This deeply enrages him. He's constantly suppressing his desire, and his father was cheating! He felt like a fool for trying to stop his feelings. If his father was going to leave their family behind, then he vows to make his family his property. 
 
That night, he witnessed his mother dozing off, and decides to molest her, but while he was enjoying his mother's seductive body, his sister witness the scene...

Kazoku: Haha to Shimai no Kyousei