Legal Question in Personal Injury in California

A person is responsible for paying medical bills only if they are the child's parents or are legally responsible for the child's injury. 5 year olds often ar not careful and injury themselves wlith no fault of anyone else. You do not give us any information to determine who might be at fault. You might even be at fault for allowing her to take part in a dangerous activity when she was not being supervised. You do not say whether there was a defect in the product. Nor do you indicate whether your have insurance coverage. Also, you must consider the practical matter of what trying jo get your sister-in-law to pay for any of the medical bills will have on your relationship with her and your wife.