Your niece has been possessed by a demon and turned into a little bitch. You have to exorcise the demon by pouring sperm that contains the power of the demon into her womb.
A curse that makes it impossible to use all of the physical strength, sword skills, and magical power along with powerful estrus unless the man's sperms are ingested regularly. The story of girls who are struck by such a ridiculous curse and travel to break it.
Stop time (time stop adventure RPG) time stop and her thoroughly of a friend. It humiliates in various places, and shames by stimulation which attacks after time releases.