Was it a dark room and they wear the same perfume?

Did you trip and your erection fall into her vagine?

No. These things did not happen.

You intentionally fucked your girlfriend's sister.

In a Penthouse letter, this is just the start of an amazing adventure that will eventually include their parents, the mailman, a brace of JW missionaries who come to the house, and a balloon.

But in real life, you are completely wrecked. When this comes out, and it will (because you and the sister are both idiots and can't be trusted to do the smart thing), their family will be ruptured and you will be hated.

Exit this situation with as much grace as possible while you still can.

Once you two make it official, I hope you have a brother so she can bang him and do exactly what you have done to her sister.

NOT an accident. You both knew exactly what you were doing and there is absolutely no way that either of you is getting out of this unscathed. Your “girlfriend” is going to be hurt more than anyone as she’s been betrayed by both of you. Her own sister??? Nice. She’s the only one I have any compassion for.

P.S. The chemistry from doing something wrong and forbidden will not last.

This was deliberate and selfish. I feel bad for your girlfriend- she is going to be crushed. Her sister is a horrible person, you two deserve each other.

You did not accidentally fuck your girlfriend’s sister. Nobody accidentally fucks anyone. You may not have given the matter any thought or it may not have been premeditated, but both of you knew damn well what you were doing and chose to do it anyway. Your girlfriend is going to find out and she will hate you and her sister. Time for both of you to grow up and take responsibility for what you’ve done. There is zero chance that you will end up happily ever after with your girlfriend’s sister. My heart goes out to your girlfriend. She is in for a world of hurt when she learns that she had been betrayed by both her boyfriend and her sister.

Super hot!! But holy fuck, what a toxic thing to do... and to think you have a chance at swapping them out... messed. Up.

I saw a similar story in the Toronto Star. A man's girlfriend moved away for years and then came back. The younger brother asked her to dance first, asked her out first and they ended up moving in together. The older brother was still hurt about it. Apparently the chemistry between the other two did last.

Hahahahahahahahahahahaha!

As a guy that's been in a relationship where I found the sister not PUYSICALLY attractive, it didn't just happen.

You both found each other attractive, you both entertained the thought, you ended up in the position to act on it, you both knew the consiquences, and you both indulged.

The fact that she still wants to continue it leaves you alone in the position to do the right thing. I never allowed myself to be in your position. I don't know what exactly the correct course of action would be, so I can't tell you what to do.

I do however know what's wrong. Don't make the same mistake you've already made if you truly care about the person your currently with. Your chance to have a relationship with her is gone, no matter how long you prolong the inevitable, but you can limit the damage.

Leave.

Best case scenario, you break her heart, and she doesn't know why. Worse case scenario, you break her heart and cause a major problem in her family.