Iranai Akuyaku Reijou, Waga Kuni de Hikitorimasu wa - Yuushuu na Go-reijoukata wo Tsuihou da Nante Orokana Mane, Kuni wo Horoboshimashite yo? - Vol. …

[Cobaltzero]

This is a fantasy story with magic and Dragons. There is literally no reason to include incest. There is nothing realistic in this series so why should it have to be realistic when it comes to Medieval relationships?

No reason not to.
Besides, if the princess has her way, she's going to be breaking up that pairing anyways.