My sister is making me uncomfortable

I'll just get straight to the point. My sister's bedroom is in between my room and the living room where our father sleeps. Years ago, when we were younger, I would swear that I could hear moaning sounds coming from her bedroom. It made me uncomfortable back then the same way it does now, so I told our mother about it, and she listened through my sister's door to see if she could hear what I was hearing, and she did. She went in the room and confronted my sister about it, but she excused it as being audio from a show she was watching. YEARS later (it kind of stopped in between), it sounds like I'm hearing the same thing AGAIN. We're all adults here, and I'm not sure what exactly she's doing (whether watching something or having phone intimacy with someone), but I'm growing really sick and tired of it. She'll just deny it if I or our mother try to tell her about it. The part that really makes me mad is that, like I said earlier, our dad is on the other side of her room as well. Like, it shares a wall—a thin wall, mind you—and what kind of lack of shame or respect do you have to have to not care who can hear? Especially after having it be known that you ARE audible.