1. You can discuss your situation and negotiate the bill with the hospital. Generally speaking, the will reduce the bill, sometimes 50%.
2. The bill, especially the doctors' bills, has to be paid, reduced or not. If you don't pay, the goverment (tax payers) has to pay. If the goverment finds out you are the sponsor of your mother-in-law, you may have to pay the full amount.
Therefore, the best solution is to negotiate with them at first.