I hope you can convince her to see a good geriatric medical doctor to rule out something physical, such as sinus problems. She may very well be smelling something real, but something that comes from within her.

Since she noticed the smell somewhere besides her home, I hope she can see that selling her home may not bring her any relief.

Maybe there is some metaphorical aspect to it as well? Perhaps she wants to not have to have a house to take care of at her age, would like to have a flat instead. But she might feel guilty or worry about making her family angry to not want the house that her husband built. Perhaps life in that house 'stinks' and she wants a change?