Sister-In-Law's Revenge

On Women's Day, the company had a half-day holiday. I got off work early and heard an ambiguous sound coming from my sister's room. The woman's voice was very delicate, completely different from my sister's. I quietly approached and opened the door a crack. A woman was naked and rising and falling on top of my brother-in-law. But she is not my sister, and I don't know her at all! This novel and its characters are purely fictitious. Any similarity is purely coincidental and should not be imitated.