Lol, (spoilers for aoi/hinata route) it is later revealed they are related by blood, aoi is his half-brother/nephew, so I suppose it is familial love (similar to Naotaro and Shinnosuke). They do care about and love each other, but it is never really portrayed as romantic (although you can interpret how you want). Currently, Sakura is the only one who explicitly stated to have romantic feelings for Aoi, and Aoi seems to return these feelings (the letter in the beginning addressing to him, being willing for him to be the one who receives the Lemures Blue, etc).