There are no hard clues as to who James FORREST's mother was, but on balance of probabilities it is thought that she was Nancy ELLISON.
There are only a few marriages on IGI that fit. In one, Nancy ELLISON married a William FORREST at St Mary's, Blackburn on 2 Mar 1832. This fit with James birth in about 1832/3. Another tenuous suggestion that James' mother was called Nancy is that he appears to have named all his seven children after his or his wife's close relations:
Firth - named after wife's mother's maiden name (FIRTH)
Robert Archibald - wife's father's first name and surname (Robert ARCHIBALD)
John - wife's maternal grandfather (John FIRTH) and/or his maternal grandfather (John ELLISON)
Nancy - mother (Nancy ELLISON)
James - his own name (James FORREST)
William - father (William FORREST)
Harriet - wife's mother's firstname (Harriet FIRTH)
No other children of William and Nancy have been identified, but we only known of James through his marriage certificate. If they did baptize chidren then the records have been lost. There was no death registration record for the child that Nancy was delivering when she died. This may be because they survived, or the child may have died in utero.
Other possibly relevant information:
On the 1871 census, a Richard ELLISON, nightwatchman at a cotton factory, was living at 12 Cross Street which was two doors down from William's son James FORREST. No link between these ELLISON families has been made.
A Nancy Ellison buried a daughter Mary aged 17 months in Blackburn on 28 Aug 1816.