I think the best believers of us all are skeptical. We should not jump to the conclusion that everything we see or hear is paranormal - that helps to confirm that spirits/ghosts/guides are real. I'd go further that just try to explain away what you see - I'd even have the wiring checked and clean the furnace vents (maybe puffs of dust) before I'd say "yeah it's a ghost".
Is your sister older or younger than you? Sometimes the younger you are, the more you see and hear. Maybe she can give you some details that you can check out - who was the previous owner of the home, was anything on the property before your house, etc.? If she discribed the older woman, maybe you could go through an old photo album to see if it's a deceased relative.
Keep on questioning - it only helps you reach the correct conclusions.