ZEX-371 My sister-in-law is a sex slave only for my father and me, Mio Ichijo

My parents divorced and I was taken in by my mother's side. - I've started a new family, and I've always dreamed of having a kind older brother. - Until then, I thought we were a normal family... - The new family has a secret that they can't tell anyone... - I wonder if I can now become a member of the family?