Yome no Imouto to no Inai Tsuma ni Kakurete Majiwaru Otto to Gimai [ Appetite, MangaGamer ] English

Handa Masayuki and Mayuka have finally gotten married after seven years together. Today, they’re moving in to their new home, and Mayuka’s sister, Natsume Sasa, has come over to help out.
“I’m okay over here, so could you help him out?”
Mayuka ask, and Sasa goes to help Masayuki. While they work, Sasa pushes up close to Masayuki. Suddenly, she says, “I’ve loved you since we first met.”
Masayuki stops working at the sudden confession. Sasa approaches Masayuki and kisses him. Masayuki tries to pull away to avoid being discovered. Masayuki isn’t in the mood, but Sasa tries to tempt him, lifting up her skirt. “C’mon, let’s do it… Nee-san’s in the other room!”
Masayuki’s lust rises higher and higher.
Just then, Mayuka enters the room. Masayuki and Sasa hurriedly fix their clothes and pull apart. Mayuka doesn’t seem to suspect anything when Sasa acts like nothing happened.
Unable to resist Sasa’s charms even if it means betraying his wife, Masayuki begins his secret affair with Sasa.
“You know how I feel, don’t you Onii-san? I’ll be direct: go out with me.”