Wasn’t there a part in the previous chapters where somebody called him a prince or something? She mentioned that he’s like an adopted cousin or something. He’s a member of the family, just not by blood. They’re cute together.
And even if they were cousins, nobility was inbred as hell. It would be completely normal for her to marry her cousin. Actually, it would be weird if she didn’t, and they would hate on her for marrying a commoner or a foreigner.