How far to push ?

Helping a chronically ill patient to deal with her daily tasks is hard work. My mother is often tired and sleepy; and does not feel like eating much. But if we let her be, she will lose even more weight and energy. We need to coax her to eat more and walk more - and this tires her out, so that she gets angry and irritable. Everyone in the family also has differing opinions about how to handle her. Is it really kind to just leave her alone ? Sometimes love needs to be cruel - but how much to push ? and when to stop ?