She should avoid crossed leg sitting(chowkdi) to avoid unnecessary friction of the implants so they will last longer.

As sir said consultation with your physician

Kindly consult your doctor who operated on your mother.

Post knee replacement surgery.. generally it is better to avoid squatting, indian toilet and stairs for long duration. Though the patient can do it, it isn't recommended as the polymer spacer in the knee gets worn out early with time . For unavoidable circumstances you can do that but avoid in general for longevity of your knee replacement.