Kusame is envious and jealous of Shizumasa, because Haine loves him.

He considers Shizumasa like a rival that he needs to knock down. But he meets Haine's opposition, since she considers him a brother and Shizumasa the beloved person!!
When Kusame realizes that Shizumasa has broken Haine's heart, he doesn't act as a childish lover, but like a man, talking with him about his feelings for her and asking him to resolve the situation with Haine's real mother in order to don't make her sad again.