Boku wa imouto ni koi wo suru
This is a very taboo story about incest and love between twins. 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?
(Description from the manga)
Audio track comes in two flavors, the difference is they have separate male lead seiyuu's. All other seiyuu's are the same, though most of their lines are separate recordings.
Info: OVA, 1 episode, on air from 18.05.2005