ヤリまん娘 ～俺の妹はビチビチビッチ～

The Miyamas were a happy household of five.
Eldest brother Kei, after his father's remarriage,
had adjusted to life with a new stepmother and two young stepsisters.

One day having lost his keys, Kei sneaked into the back door of the house
and spotted Shiho stealing his underwear.

He was like a deer in the headlights, but she was unflinching: "I want sperm," she said.