I am sorry to hear of this tragedy in your family.
Unfortunately, no physician can accurately predict how long any patient with a terminal illness will live. Other than to try and keep her comfortable, and to encourage her to eat and drink, there is not much that you can do beyond what you are already doing.
You might want to look into hiring a home hospice service to help you to care for your mother-in-law. As you already know, caring for a terminally ill family member by yourself is an enormous responsibility.
Sincerely/ Robert A. Wascher, MD, FACS
http://doctorwascher.com