Who is liable/responsible for medical bills if a parent falls and breaks their leg on a school sidewalk?

My sister was walking with her son into his school around 8:30 am after a couple inches of snow had fallen. The sidewalks had not been shoveled at all. Her and her son both ended up falling even after acknowledging the slick condition. They both got up and my sister could barely stand or walk, but took her son into the school anyway and told his teacher what had happened. Apparently the woman that normally clears off the sidewalks was off that entire week due to a death in the family and there was no one to fill in for her. The principal has told my sister they can't help her with her medical bills in any way. Is this true? Her leg is broken and also may have torn something that requires surgery.