It was mentioned that Zenith was taking care of his baby sister who was down with a fever. They might have magic but diseases can still wreck a small infant.
Honestly the harem side of this was probably gonna turn me away but I don't mind it since there is an actual in story explanation for it. It does actual mimic the insanity of medieaval politics and inter family political marriage quite well. Still find it creppy. (Especially after glancing at the wikia)