This isn't even a political marriage if it's two branches of the same family so close they're literally cousins. That's not forming alliances with anyone new, that's just a pedo who loved his cousin since she was a little girl and her father just wants rid of her.
I love how culturally confused this is. Though that's par for the course. If pervert and lolicon wouldn't really come up of this is supposed to be based on medieval European culture. But then again if it really were these people would be dying in their 30s and women after childbirth. I don't know it's just kinda funny cuz these is every vaguely medieval esque fantasy setting. Just a mix of cultural values to the point where you can't really tell where the civilization stands.