Your sister will have to appeal to her son on moral grounds. The law, unfortunately, will not be much help. In general, once someone has relinquished property with a signed deed, the new owner has the right to do with the property as he or she pleases.

If there are special circumstances that make your sister think she retains some right to the property, or if she was somehow incapacitated when she quitclaimed the property, it might be worth her while to consult with a lawyer.