What should i do if my sister-in-law hit my daughter for vomiting?

i have two daughters named bella and penny. penny, my younger daughter, suffers from leukemia. one day bella had a ballet recital and i went to watch her perform. penny was not feeling well that day so i left her with my sister-in-law, roxanne. penny got sick and vomited all over the rug. that rug was roxanne`s favourite and she got really angry. she gave penny a brutal beating and punished her severely. that incident was so traumatic that penny ended up in the hospital. how should we solve this problem?