I went through Tweedmouth Parish baptisms 1815-50 for Johnsons of Spittal afew years ago and didn't note an Isabella but there was only one of Benjamin and Barbara's children, Margaret in 1834 although they had three children in 1841 census. There seems to be missing baptisms at this time and many marriages are not recorded as they often occurred at nearby Border crossing points. If she was married by 1841 then no clue as to parents in the Census. Naming patterns were very strong in the area so can you tell me her children's names in date order as that may provide a clue as to her parents.