Ahh, of course there's an age gap... There can't be a manga with anyones romance characters being of similar age, that would be icky right?
I'm so tired of such large age gaps...
Also "you're like a little brother". Then could you not be so sussy toward him and try to seduce him? This can be a political marriage without that ya know, He would have understood. His 13 for lords sake, have some class.