From Awakened Memories and Covenant of Darkness:
My private tutor, Midorikawa-sensei is a student attending a super elite medical school. He confessed to me and we started dating but I inadvertently lost my virginity!
It is because, everyone around me already experienced their first time and I thought it was too late for me.
At that time, my non related sister who has always lived separately from me returned. We met for many days and suddenly she is a he. A he who wants to have a romp in the hay with me!?