She can go for hip replacement
first followed by knee replacement
,considering she is medically fit. If not fit for some or other reason, get her fit by consulting a physician & get her hip & knee replacement as soon as possible, since she is 76 years old & delaying these surgeries will affect the results, adversly.
I am a joint replacement
surgeon doing joint replacement with smaller incision & latest technique.
You can contact me at firstname.lastname@example.org