Stepsister or Girlfriend?

I thought making out with my stepsister would just be something fun and chill—she's so inexperienced and always dresses like a little girl, so I figured I should show her what a man tastes like, but it was only a kiss! Now she is freaking out because I brought my girlfriend home, and she won’t stop rubbing my cock with her feet underneath the table... in front of my girlfriend! She is so jealous that she is even dressing differently and acting like a slut. Where is my innocent, little stepsis? Too bad I’m too weak. There’s no chance I’m not going to fuck that ass if she keeps provoking me or touches my dick one more time, which I know she will do.