I agree with you on everything you said except, the way I interpreted the story was, Kanata originally loved Hisakuni (as kids) but felt guilty for causing the accident that cost him his eye. That guilt made him consider himself unworthy to love Hisakuni, but it didn't make him stop loving him. I got the impression Kanata "tolerated" Yuki (both having sex with him and his carousing) in order to stay close to Hisakuni and to keep the only family he ever knew. After Yuki's death, another accident that Kanata feels at fault for, Kanata is now feeling unworthy to love or be loved by anyone. Guilt, now as before, is preventing him from seeing the truth of his love for Hisakuni and the love Hisakuni has always held for him. Anyway, that is how I read and interpreted it. Your statement about love is "on the mark" IMO though. The heart (love) is the only thing that gets bigger the more you give it away. (⌒▽⌒)
The heart has no boundaries it can love one and more. I think Kanata loved both but he loved Yuki just a bit more so meaning Kanata chose Yuki this is why The brother stepped down and just ...was there(●￣(ｴ)￣●)