Hot Summer Job/Slip your finger inside your sister
You allow your fingers to drift closer to her grinding pussy, gently parting her labia with your middle finger. She seems to miss a beat for a second in her grinding but she doesn't make any indication something's wrong, perhaps dismissing your roaming fingers over her bald pussy as a possible accident due to your very compromising situation. Her gyrating slows a little, but it does continue. In fact, she seems to actually be trying to track your fingers a little now, pleasuring herself against your “conveniently” placed appendages, and in her use of your didgets to get herself off you have little problem fitting in with the motion of her hips in order to get the first segment of your middle finger past her virgin entrance.
Your entire first digit is inside her now, and the tip of your finger is pressing up against her hymen. You can feel the fleshy membrane pressing back against you as your finger nail pokes through the tiny hole in the middle.
Now she definitely notices. She suddenly stops and her eyes dart straight up to you. They are not accusatory, more surprised. Maybe you could still get away with passing it off as unintentional.
Play oblivious
Be bold