So the prince is related to the blond mage (cousin) from one of his parents. Let's say Parent A. Parent B is related to the Diarmuid (The tower master mage dude), so the blond mage is also cousins with MC. Jaina, the MC, is not related to the prince. Hope that helped!
I think the blond haired mage (Jaina's cousin) , his mother was as I remembered was Diarmud sister and his father is of royalty or vice versa. So the Prince and Jaina are unrelated to each other.
siblings or dating final boss
I mean at this point we can tell they really aren't siblings