Koisuru Futago to Megane no Blue

Momo and Sakura are twin sisters that are extremely close. The older sister, Momo, has been sickly since she was a child. The younger sister, Sakura, is a lively and energetic girl. Their personalities are complete opposites, but they have always lived their lives by splitting everything in half. That way, she can enjoy the world twice as much... One day, Sakura's shy older sister, Momo, falls in love for the first time, and she is determined to support her with all her might. But the man she's in love with is...?!

(Source: Kodansha, translated)