Kimi ga Boku no Mannaka

Steady and caring Yuzuki and clumsy and spoiled Miumi are unrelated sisters. They've always gotten along like best friends, but one day Yuzuki meets someone she's interested in. Yuzuki is steadily getting closer to him and is filled with happiness. But Miumi can't help but worry...

(Source: Shogakukan, translated)

Note: Includes two extra chapters.