Not finding a suitable marriage online between Benjamin & Elizabeth in area, maybe they did not marry? Also considered she may have married previously or had children with another.
Think this baptism may relate to Elizabeth. Some younger children post reg have mothers MS as Nicholas.
Transcript only, no image - maybe try familysearch ?
Elizabeth James
[Elizabeth Nicholas]
Baptism Date 09 Oct 1808
Baptism Place Brawdy, Pembroke,Wales
Father James Nicholas
Mother Alet
FHL Film Number 105130
Not sure if she is illegitimate and her mother is surname James? Maybe her father went by both names? I did think of Patronymics, but ruled it out as a bit late.
Also in 1841 there was a Francis James age 1 in household with family. Did not find him under surname James, but there is this possible. No mothers name listed.
NICHOLAS, FRANCIS -
GRO Reference: 1840 J Quarter in HAVERFORDWEST Volume 26 Page 619
Cannot be hers as was having Thomas children around this time. Possibly just a family member if any relation.
Bit of a complicated family
Cas