I think this below may be the Mary with William in 1901, however from thecensus information , I would think Williams mother was more likely one of Charlottes other children?
MORTON, MARY BEARDMORE
GRO Reference: 1876 M Quarter in ROTHERHAM Volume 09C Page 621
1881 Catcliffe Common, Catcliffe, Rotherham, Yorkshire (West Riding)
Andrew Morton Head Married Male 46 1835 Glass blower South Shields, Durham
Charlotte Morton Wife Married Female 45 1836 - Worksop, Nottinghamshire
William Morton Son Single Male 23 1858 (soldier) army Masbrough, Yorkshire
James Morton Son Single Male 19 1862 Glass blower Kilnhurst, Yorkshire
John Morton Son Single Male 14 1867 - Catcliffe, Yorkshire
Herbert Morton Son Single Male 11 1870 Scholar Catcliffe, Yorkshire
Walter Morton Son Single Male 8 1873 Scholar Catcliffe, Yorkshire
Mary Morton Dau Single Female 5 1876 Scholar Catcliffe, Yorkshire
Above in 1891
https://www.familysearch.org/ark:/61903/1:1:7L18-96ZCharlotte in 1911
https://www.familysearch.org/ark:/61903/1:1:XWKR-HT5Colin