@Huapollon
This of course precludes the idea that's if such a scenario where to transpire their children wouldn't end up being allies as opposed to enemies, because if the brothers have his type of relationship as they do, likely the children will as well, and as for who gets the throne, it sounds like it is partly being chosen by merit as opposed to who is in first position, so they may end up in a situation where if such a marriage to take place, the two families couldn't earn fight it out not by War, but by mutual support, since by having memories from the future, the MC has the ability to offer a different perspective on how the world might operate—likewise, one of them might choose not to pursue the throne because they think that the other one or other ones are in a better position.
As for Grandma's family, it appears as though quite a number of them are going to be executed in the near future, so it would seem that after The Purge, it would end up being Blondie and brunette starting new family's accordingly.
By this I mean that since the current head of the Parlams has set about his own execution by his nephew & his nephew's brother, it would seem reasonable that since Noah will likely in turn take over the Parlam estate, if he married Gracy, his & her kid could form a new estate combining the two, thereby offsetting the crown, or enable whichever kid to rule based upon "qualifications" —Canaria's future memories are as yet underutilized. Likewise, treason typically meant the deaths of 3 generations of a family. Hence, why the US did away with "Blood Guilt" where you'd kill everyone in a family for 3 generations of obe member committed treason. Noah would likely be spared as he's royalty and was in-league with the crown (helping foil the traitors), but his mother, uncle (and his family) would all likely be put to the sword.