Forbidden Sex with my Sister's Girlfriend

There are some things in life it's probably not a good thing to do, one of those is to have sex with your sister’s best girl friend.
It is not only a taboo thing to do, but it also leads to so many problems it's not worth it.
I knew all this.
Which is why one night I threw all my experience out the window and slept with my sister's best girlfriend.
Why not be a voyeur, and see what happened!