It Was A Prank, But Now I Want To Fuck Her Again

Since this happened it's been a little awkward between us. Not in a negative sad way, but awkward in the sense we both know we want to do it again.

With our parents away for two nights celebrating their anniversary, I invited my girlfriend round.
My sister Becky and my girlfriend had some wine. Then pretty late on I got a phone call off a friend of mine. I'd taken a set of his keys home in my car. I drove over to give him his keys, then drove back, but id been gone longer than I said I would.
When I got back the house was in darkness, so I just thought they'd gone to bed.

Feeling horny for a fuck, I got naked after walking into my bedroom and sure enough my girlfriend was lay facing away from me.
Getting in with her, I didn't mess about and eased my already erect cock, straight into her as we spooned.
Reaching around I played with her breasts as I fucked her harder and harder, then I heard and felt her orgasm on my dick. Only then did I realise it wasn't my girlfriend but my sister, whovis a similar build and has long blonde hair like my girlfriend.

She spun around, told me to be quiet and got out of my bed.
Then putting on her PJ's which I'd not seen in my rush to have sex, she began laughing loudly. Moments later my girlfriend came in and they began laughing together.
It had been a prank, but one where I wasn't supposed to fuck my sister.
Becky made out she'd turned around as I'd stripped off, and didn't say anything about me fucking her, so I didn't say anything.

Becky went to her bedroom and my girlfriend got in bed with me.
We had sex, but id not been able to clean myself, so my girlfriend sucked my cock with my sisters juices on it.
When my girlfriend left the following morning, I asked my sister why she'd not stopped me. Her response was "I've seen your cock before, and wanted to know how good it felt".
She didn't apologise at all, and instead since then has been flirting with me.

I have to admit I'm desperate to fuck Becky again, and I know for sure she wants my cock, but we both know it's wrong.
Only yesterday she invited me into her bedroom, saying she wanted to "suck it". I refused but it was extremely difficult. If my mum hadn't have been home, I think I would have fucked my sister again.