As per the age mentioned, your mother in law must be having the age related anxiety too that may be one of the reason of mood changes n nervousness. Talk to her n make her understand or motivate her that people do get sick n by medicines they do well also. If possible keep talking to her as n when u all get time that will help her to make that she is not alone.
Next Steps
You can arrange a tele Counseling for her that will again help her ventilate her emotions well.