Ahah.....Sarah MARTIN in 1891 is a red-herring and is actually part of the next household.
The address of the next household, 15 Anderton Street - is next to Sarah's name (i: on the same line), then is listed after her, at that address also is:
Head: James Cartwright 34,
Wife: Jane Cartwright 30,
son: Henry MARTIN 8
Son: John Thomas MARTIN 5
In 1901;
Head: James CARTWRIGHT 41,
Wife: Jane CARTWRIGHT 40,
Stepson: Henry MARTIN 18
Stepson: John Tho's MARTIN 15
Mother-in-Law: Sarah MARTIN 68, widow
So....that's the end of that line of thought

Cheers
AMBLY