Nancy and Albert are on the 1880 census at the home of their uncle John. Their mother Elizabeth (John's sister) had died in 1866.

1880 US census St. Clair, Columbiana, Ohio, p 629:

John MONTGOMERY Self S Male W 34 OH Farmer PA OH
Nancy MONTGOMERY Sister S Female W 39 OH Keeping House PA OH
Albert MCCOY Nephew S Male W 17 OH Farm Worker OH OH
Belle MCCOY Niece S Female W 15 OH Keeping House OH OH
Julia SLAUGHFMAN FHMAN Other S Female W 30 OH Housekeeper GER GER

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Source Information:
Census Place St. Clair, Columbiana, Ohio
Family History Library Film 1255002
NA Film Number T9-1002
Page Number 629B