My aunt age is 50yrs. she is feeling right knee pain when she sit on floor and stand up. she feels some rigidity at knee. no other isuues.

Probable minor: This could be caused by several minor issues in the knee. Sitting on the floor puts a lot of strain inside the knee and the cartilages in the knee. At her age she may also have early arthritis. If there is no swelling it is probably a mechanical pinching inside the joint. If there is swelling could be more of an injury mechanism involving the muscles and ligaments outside the joint.

Meniscus tear.: It could be a meniscus tear, arthritis, or patellar pain. Hard to be more specific. She should see an orthopedic knee specialist.