I think, because of the nature of their relationship, its very difficult to put a label to it? It's definitely much more than a simple friendship or familial love or even romantic love

They were friends, they were brothers, they were the same, they were confidents, they were each other's support, they didn't really knew each other and were too self absorbed to learn, they didn't want to get separated but they did

A part of Hikaru died with Sai