Before getting married a woman knows her future husband was sterile. She had been greatly saddened because she had longed to be a mother biologically. Her soon to be, comes up with a plan. After the wedding, they'd go to a special room and one of the guests would take her virginity and impregnate her. They agreed that she wouldn't know who it was at the time of the sex.
After the baby is born she wants to know who the father of her baby was. When she was little she had been adopted and could barely remember life before her new family. And one person had been trying to get in her pants since a few weeks before her 13th birthday. The father of her baby is that person. Her adopted brother.
After she finds out it's agreed that she and her brother can make more babies. And her affection towards her brother grows.