A third voice; Shannon's sister

"I can't wait to have that big cock inside me again," I hear a female voice say.

That wasn't Shannon's voice, but it does sound familiar somehow. It sounded sort of like Shannon's little sister Mary, but it couldn't be her, right?

"You know the rules Sis," Shannon says, answering my question, "If you want Mark's cock then you need to shove that sweet little tongue of yours up my twat."

Shannon is having a threesome with her own sister? She never told me anything about being interested in girls. And to be with her own sister on top of that.

Shannon's moans are coming through the wall now , so I know that Mary must be eating out her big sister's pussy. My cock is starting to get hard listening to the incestuous act.

Mary is more or less a skinnier version of her big sister. She doesn't quite have Shannon's killer curves, but she's still a sexy girl.

I can know hear masculine grunts, which probably means that Mark is now fucking Mary while she eats Shannon out. My cock is rock hard now and my pussy is really wet. What I'm picturing going on on the other side of the wall is super fucking hot. I'm really tempted to take a peek and see how my imagination stacks up with reality.