I think I get what's happening here. So back in high school brother had a crush on June. Reacted horribly to getting rejected. Destroyed the relationship.
June goes to college and thinks he reconnects with Eunsang but meets Eunha instead. June thinks he's leading Eunsang along for revenge but starts falling for Eunha.
The twins have remarkably similar taste... And the reason Eunsang hates his brother is because he's less academically smart and their parents are constantly comparing them. So he acts out. Now his old crush likes his brother better and it's fuc**** him up mentally.