Sexually ofcourse.

Standing behind her, your cock finally started to get bigger while you soaped up her back before continuing with her breasts, causing your sister to moan out. The moan sounded fake though and you barely touched it, "You know that you don't have to fake it."

"Awww, but didn't you get nice a horny when I moan like that?" the partial limping of my cock to her voice and the previous moan made her let out a grunt of annoyance, "Okay then, I have a sexy voice. And you will now fuck my asshole, brother." the new voice caused a reaction from you and place your enlarged member against the backdoor of your sister before slowly easing in.

The feeling overwhelmed you and from the sounds of it, your sister as well. Grunts came from your mouth as her ass closes and warms your cock as you piston up and down, your sister placed her hands against the wall and pushed against you, bringing you constantly closer to cum. You were a virgin before this moment, so it was a new experience for your new and much more sensitive cock.

The motions and moaning from your sister made it clear that this also the case for your sister , despite the fact that your cock that shouldn't fit so well in her asshole if that was the case. Despite what porn told you, you knew that having such a large dong was more of a problem since humans weren't made to take such a big packages. And yes, you were thinking of this specifically so that you could last longer, you had some pride as a man.

"I'm cumming! Cum with me!"

Well there went that plan as your body reacted to that command and started to convulse as you started to cum into her ass, it kept on shooting for what seemed to be an eternity for you. Even masturbating you could hold on longer for a better release because the build up was better.

"Clean my hair, we need to get ready for... the place we will be going." that was ominous, you try to pry your dick out of her asshole, "No, no... keep it in." and now your cleaning the hair while still inside of her, it was an odd experience.