Kazoku ~Haha to Shimai no Kyousei~

After his father's remarriage, the protagonist gains a non-blood-related family: older sister Nagisa, younger sister Fuuka, and mother Otoha. Though the relationship is amicable, he struggles with growing lust toward these beautiful women. One day, he catches his father cheating on his stepmother. Enraged by this betrayal, he abandons his own restraint and vows to claim the family as his own. That night, he molests his sleeping mother—only to be witnessed by his sister.