April fools prank turned into me fucking my girlfriends sister [M/F]

I'm not a great writer but I need to share this with someone

I have been with my girlfriend for seven years now, since we were 16. We have a great relationship and we have been living together for 3 years. Throughout these 7 years I have gotten to known my sister-in-law (2 years younger than me and my girlfriend) very well. We have very similar humor and I sometimes even find myself getting better along with her than with my girlfriend. At every family gathering we have always lightly teased/hazed my girlfriend together.

She came to stay at our apartment for a week back in the end of March this year, as we live in different cities. Everything was normal as it always is until April 1st, the day before she was leaving. My girlfriend was at college so I was alone with my sister-in-law until my girlfriend finished her classes in the afternoon.

An hour before my girlfriend was supposed to come home, my sister-in-law remembers that its April fools and she wants to prank her sister when she comes back. We had spent a good 30 minutes thinking and suggesting pranks to each other while watching TV on the couch when she suggests that we pretend to sleep together once she comes back.