Second Queen, Elena, is basically the cause of this entire arc. Everyone knows that the girly prince is the one who is best to take the throne, but because his mother is a gigantic bitch, it complicates matters. Especially since her son likes the First Queen and her son. So if she wasn't interfering then she would have obtained the very thing she wished for much easier. Honestly this all could be resolved by the son taking the First Queen as one of his wives after his father dies. It wasn't unheard of to take care of a deceased relative's wife at the time. Plus she's young enough. Just make Teresa (the boyish girl) your second wife and everything is resolved. Both the First Queen and Teresa who is being adopted into royalty should understand the situation and it keeps Pedro, the third son, safe from Elena's interference.
The bigger problem that can't be easily solved is what are they going to do about Margotte? She obviously doesn't want her son to be a philanderer with multiple wives. It's the main reason why she sent Sei with him to the Knight school in the first place. Now he has, Seilurn (maid girl), Selina (merchant dog girl), Rachael (princess diarrhea), Urakka (admiral boobs), and possibly Silk (knight girl) and Rorona (merchant secretary who is probably okay with being a mistress). I can't see mom being too happy about that development. Plus he's still only 13 so he has yet to get a woman that's younger than him.