Your mother seems to be suffering from Osteoarthritis
and possibly Osteoporosis
as well. What you could do is encourage physiotherapy
in various sorts to help regain the muscle power. And that my friend is the only long lasting solution. Otherwise she could take anti inflammatory medicines(not recomended over a long period), local ultrasonics/swd, visco supplements for knees, and eventually joint replacements. Do advice her on keeping healthy diet consisting of leafy vegetables, milk n milk products and fruits.
Mail me her status on firstname.lastname@example.org for more info