Okay so Yasuhiro and Hinote are brothers and they're friends but Hisashi and Yasuhiro decided to take it to the next level and decided to become a couple so Hinote is just a bystander.
One day, Hisashi was cursed by a god and transformed into a bird demon. Yasuhiro managed to tame Hisashi with the power of love and friendship and this impressed a nearby goddess and she fell in love with Yasu instantly.
The goddess and Yasu got to know more about each other and decided to marry. Hisashi was obviously heartbroken by this and Hinote became jealous because Yasu is the one getting lucky with love while his single ass is rotting in the corner.
All of this made their relationships with each other very messy and that's all. :)