I Fucked My Girlfriend's Stepsis in the Bathroom When She Went to the Supermarket

My girlfriend makes me really happy, she's such a sweet beautiful girl and she takes such good care of me in every single aspect of life. That's why I've been feeling so guilty lately for being SO IN LOVE with her little stepsister Lynn... she's even sexier than my girlfriend and I'm pretty sure she likes me back because she acts a little slutty when I'm around... one day I couldn't take it anymore and surprised her in the shower when my girlfriend went to the supermarket and left us alone...