RA factor being normal, the chances of having rheumatoid arthritis is low. However, in sero negative arthritis (incidence is low), the tests may be negative in spite of your sister having arthritis.
Next Steps
Best situation would be to get Anti-CCP (blood test) done. This test is more specific for RA.
Health Tips
Do consult a rheumatologist for better evaluation and the correct tests. I would strongly suggest you visit Dr Krishnamurthy in Chennai Meenakshi Misson Hospital.