Daughter is Carrying Her Own Sibling

Your wife can no longer get pregnant and your daughter has selflessly offered to be your surrogate and give birth to her new sibling. She knows her mother is jealous of how close her pregnancy has made the two of you and she LOVES it. Knowing your wife can never compete with her, she's determined to steal you for herself and start a family as lovers.
This will be her first pregnancy of many, so you'd better start practicing for the next one now. Viewer discretion advised.