playing doctor

my son and my sister are only about a year and a half apart and it shows. they are the best of friends. i spend most afternoons at my parents house, because they can't stand to spend a day apart.
unfortunately, my sisters new favorite game is playing doctor. her favorite patient or should i say victim, is my son of course. just the other day i overheard her say, now open up and say ahh! of course i raced in just in time to see her putting a toy in his open mouth. when i told her not to do that she assured me she wouldn't, but i'm not that stupid. so, i waited as she returned to her checkup. sure enough, within five seconds the toy was deposited back in his mouth. this isn't the first incident. one day i caught her holding his eye open, and flashing a light in it. then there was the time she tried to put a pen in his ear. my parents even tried buying her a baby doll that came with it's own doctors kit, hoping she would doctor the baby and not my son. it didn't work. now when she says, let's play doctor, all of us inch closer, just waiting to save him from the well meaning, but very dangerous little doctor.