Boku wa Imouto ni Koi wo Suru

From ShoujoMagic:

Yori's attitude has changed. He and his twin sister, Iku, used to be inseparable as children, but lately Yori's been different -- eager to be apart from Iku. Iku worries that he hates her, but the truth is far more scandalous. Now, Yori would risk losing everything to have his little sister. Iku loves her big brother enough that she would never risk losing him, and so she'll do what it takes to keep them together... but Yori doesn't want to have her that way -- as a sister dedicated to her twin. Can Iku ever love Yori not as a brother but as a man?

Boku wa Imouto ni Koiwosuru received the Shogakukan Manga Award for shoujo manga in 2008.