Soukoku 4

Dazai and Chuya are 17 years old, both study at the same school and in the same 11th grade.
Chuuya often takes Dazai home while his parents are at work late, so no one knows he's in a relationship with a guy.
Nakahara also has a sister, Mizuki Nakahara, two years younger than him, but she's quite the snitch. She's always snitching on her older brother to their parents. She'll catch him smoking, or running away from home. But Mizuki often attends various clubs after school, so Chuuya isn't too worried about her seeing him with Dazai.
And so, one day, Chuuya comes home, bringing Osamu. And suddenly, Mizuki meets them at the door.

• Wow... Are you with a friend today?
  Chuya immediately noticed how his sister started making eyes at his boyfriend, but he silently bit his lip, trying not to show his jealousy.
  “And I’m Mizuki,” his sister said, holding out her hand to Osamu, fluttering her long eyelashes.