According to the OPR index on Scotland's People William Duncan and Margaret Jamieson had seven of a family, born in five different parishes
George, born Keithhall and Kinkell, 1803
Mary, born Fintray, 1804
Anne, born Fintray, 1807
William, born Kinellar, 1812
William, born Newhills, 1814
Robert Alexander, born Banchory-Devenick, 1820
Margaret, born Banchory Devenick, 1825.
So there are plenty of possible parents for nephew George, who registered his uncle's death in 1870. It looks as if nephew George may have put his uncle's mother-in-law's name on the death certificate by mistake instead of his uncle's mother's name (his own grandmother's name).