William is Janet Cants too so it is a case of Janet and Jessie often being the same name.
John's family in 1851
MASTERTON, Janet Head Widow F 50 1801 Seaman's Wife
Boness, Linlithgowshire N/A
MASTERTON, Janet Daughter Unmarried F 18 1833
Torryburn, Fifeshire N/A
MASTERTON, Manney Daughter Unmarried F 16 1835 Out Door Labourer
Torryburn, Fifeshire N/A
MASTERTON, Mary Daughter Unmarried F 14 1837 Out Door Labourer
Torryburn, Fifeshire N/A
MASTERTON, William Son Unmarried M 10 1841 Scholar
Crombripoint, Fifeshire N/A
MASTERTON, Gorgenia Daughter F 9 1842 Scholar
Crombripoint, Fifeshire N/A
Piece: 458
Folio: 44
Page: 6
Registration District: Eastren
Civil Parish: Torryburn
Municipal Borough:
Address: Crombiepoint, Crombiepoint, Torryburn
County: Fife