Did I tell you I'm friends with benefits with my best friend's little sister?
How did that come about?
My girlfriend cheated on me and my best friend's little sister comforted me.
Having a friend with benefits is fine but are you sure your best friend's little sister is the right person to do that with? If your best friend finds out I'm pretty sure it's going to damage your friendship and you might not be able to fix it. I would probably end that relationship soon if I were you.
I'm not worried about it. If she wants to date me then that's her decision.
That could be tricky. Your sister might not feel comfortable with that relationship when she finds out. Are you willing to take that risk?