My sisters arm joke for kids

I work in a hospital giving tours to children. One day a group of kindergarten kids came in for a tour, I showed them the x-ray machines and asked them if they ever had broke a bone?

One little boy raised his hand, "I did!" "Did it hurt?" I asked. "No!" She replied. "Wow, you must be a very brave girl! Which bone did you break?" I asked. "My sisters arm!"