15-year-old Millie has an unused pussy and two brothers. The elder is already keeping her Mama happy, and now the younger wants to find out what fucking is like. Millie obliges him, and later her older brother, but her real aim is to get pregnant with Daddy's baby. Or maybe Uncle Biff's, because he's promised to show her how on her next visit. Millie and her Mama are willing receptacles for all the sperm the men in the family want to dump in them. For sure they'll end up pregnant.
"Yes, dear, and one day soon I must tell you the purpose which menstruation has in Nature's creative plan -- but not today." [Callender, Mary Pauline (1938) "Marjorie May's 12th Birthday", Kotex. Co., p. 12.]