Your parents have been trying to have a baby for a year now. The empty nest has been hitting them hard. You find mom getting a call, she’s healthy. It’s your father who’s the problem, even if he refuses to admit it.
You offer mom your help where you can. Which she denies until later in the night. After a bath and a think she decides you can help. Help by fucking a sibling into her belly.
She’ll keep it secret, all you have to do is agree… and cum.