Two complicated yibum situations, with regard to the co-wife, and whether there are other relationships that would exempt the co-wife. Say, the sotah. Or a divorcee. Note: "Sotah" in this example means a woman who inadvertently committed adultery, and not the usual definition of a woman who is suspected of having committed adultery. And divorcee here is specifically remarrying her first husband after having been married to someone else.