Loli no Imouto ga Ore ni Seiteki na Shisen o Muketeru Ki ga Suru.

Kotone Utano loves her big brother Hibiki.
It's taboo to ask him what she wants to ask him and it stresses her out,
but when their parents leave them home alone on an overnight trip,
she seizes opportunity to become fast intimate with oniichan.
She refers to herself as "Kotone" but, her inner heart she says "I".

Hibiki Utano is Kotone's big brother.
A student. Young with a powerful, almost unmanageable sex drive.
Thinks his little sister is cute but, not yet awakened to sexual love.