You mention that she fell with a fracture to her knee and had a stroke a year ago. The weakness she is experiencing may be a residual of the stroke or an indication of some other condition that is going on. Be sure to discuss your observations with her doctor. Ask if there is any medical condition that may be contributing to the weakness. The doctor may want to do some blood work or other tests to see if this is a new problem.
I would also suggest you talk with the doctor about physical therapy for your mother. If the doctor does order this therapy you could accompany your mother to the sessions and see how therapy is done and get a plan for continued exercise after therapy is finished.
As for the actual exercises, while weights can be added to the routine, you should not start with them. Start slowly to reduce the risk of injury or fatigue.
