Cailan was sterile because Queen Rowan was tainted while she was pregnant with him. Rowan was tainted during her time in the Deep Roads (The Stolen Throne). She came back with the taint which eventually killed her (the wasting sickness she died from sounds eerily like succumbing to the taint). Cailan was born free from the taint, but it did sterilize him. There was/is nothing wrong with Anora’s fertility.