"Both in the bed"

"You're right", I said. "There's nothing wrong with that."

We cleaned up and got ready for bed. Since my sister didn't have her sleeping clothes with her, she wanted to sleep in her underwear and I decided to give her a shirt of mine, so she wouldn't have to wear her uncomfortable bra. I had no problem with my sister sleeping barely clothed, the only thing worrying me was that my dick might stand a little firm and Cassie might notice.

After changing into my sleeping clothes (a shirt and boxers), I finished up in the bathroom and went to bed, leaving the bathroom to my sister. She followed to my shortly after. My shirt wasn't long enough, barely reaching to her panties (I might have given her one of my smaller shirts) and the sight of her like this got me quite excited. As she turned around to close the bedroom door, I got a perfect view of her ass. The way her panties were embracing her firm buttcheeks was so damn sexy. My dick hardened. I turned around just as Cassie did, so she wouldn't notice me looking.

My sister was lying down next to me, I had my back turned towards her, not wanting my hard cock to point in her direction.

"Hey, John", my sister spoke up. "Can you like, put your arm around me, please?"

"Uhm, yea, sure, no problem."

Like hell it was no problem. I positioned my crotch very carefully, because I didn't want my dick poking my sis in her back, and then put my arm around her. Cassie pushed my arm up right against her underboobs. My dick got even harder, pushing slowly against my sister's ass. I was hoping she wouldn't notice.