Mama wa Joshikousei

My father got remarried with a school girl and then suddenly passed away.
I continue to live with his wife, who is also my student in the school where I work as a teacher.
One day, she found my favorite porn comic about mother-son incest relation, and naive as she was, she believed that it's part of the mother's job to offer sexual service to the son!

[From English DLSite]