Parasol Doumei

A boy, one of twins, struggles to keep up with his high-achieving sibling in school and social standing. He asks out a girl, also part of a twin set, mistakenly believing her to be her sister. After a mortifying misunderstanding, he finds himself in a difficult situation and makes a deal to assist her in pursuing his brother. In exchange, she agrees not to reveal his failed proposal to her twin, which could damage his chances with his own crush.