Despite what prom would have you believe, most girls are not turned into a hot mess by the sight of a big cock. If I've learned anything, it's that it can be alienating and gross to see someone's junk unless she's in the right headspace for it. So, this isn't some porned up fantasy about how my towel fell off and my sister jumped me or whatever.

What did happen was this. I came home from uni early one term because lectures were cancelled on account of a strike. I got into the house, but when I called out no one answered, so I assumed there was no one there. Anyway, I went upstairs to dump my stuff in my room when I noticed the door to my sister's room was open and there was some sort of movement inside. I'm not 100% sure why I hesitated, but I crept up and looked through the crack the door made against the wall.

My sister was on her bed with her laptop, clearly masturbating. I was shocked at first; it felt so weird. I didn't know what to do as doing nearly anything felt like a mistake. So I kind of just stood there. It was slightly more disconcerting to see the the porn she was looking at: the voiceover made it fairly clear was incest stuff. The weird (or maybe not so weird) thing is that I always thought that was my kink. I suppose that was when my mood sort of shifted into being aroused by the whole thing. Not so much the thing itself, but the taboo of it. It played on my mind for days afterwards, and I when masturbated to the thought of it, it made me orgasm so incredibly fast.

I found myself leaning into the clichés: all the stuff about showering with the door ajar, and coming into her room in a bath towel pretending to borrow a phone charger and the like.

Eventually, I make the 0/10 originality move of 'accidentally' sending her a selfie of me with a bulge in my shorts. After the obligatory 'OMG! Meant for someone else!' I waited for the reply. "Haha, don't worry! Not yours, don't believe you" was the answer. "Check if you like!" was all I could say back. I just got a smiley with a wink to that.

I guess from there it became a game of chicken: each of us flaunted ourselves at the other when we were at home together. She'd wear a t-shirt with no bra and lie on my bed next to me; I'd be sure to exit my room with the same shorts as the photo––and the same bulge. When we finally crossed the line it was after we'd both smoked some weed and decided to see who could give a better massage. I still remember the feeling of her softness under my hands, and the frisson as she slid her hands under my waistband and touched me so gently.

We never had penetrative sex, but it doesn't matter: I still get aroused thinking of those days. Are you a sister who crossed the line with a brother or other family member? Talk to me and we can compare notes!