Do you know where Mary was born? Just that there is a Nathaniel with mother Mary (a widow) at Aberdwfr, Caron-Ys-Clawdd Aka Tregaron, Tregaron in 1881....
Mary Jones, Head, Widow, Female, 29, 1852, Coal Miners Widow, Llanddewibrefi, Cardiganshire
Nathaniel Jones, Son, Single, Male, 5, 1876, -, Cardiganshire
David Jones, Son, Single, Male, 3, 1878, -, Cardiganshire
Margaret Jones, Daughter, Single, Female, 1, 1880, -, Cardiganshire
RG11, Piece number 5452, Folio 50, Page 8
This ties in with what you've found in 1891 for Mary and Margaret.