Thomas Morrow is shown as the father of Sara, Anne, John and Isabella. This may or may not be correct. For her second wedding Isabella gives her father’s name as Thomas Morrow. In the 1851 census Sara, Anne, John and Isabella are listed as siblings.
1) One possibility is Isabella’s mother, first name unknown - last name Blythe was not married to Thomas Morrow. In this case the mother likely died before 1841 and Thomas Morrow had either died or was not in the picture. Isabella was being raised by much older siblings, who may be half siblings - same mother different father or fathers.
2) A more likely possibility is Isabella is the daughter of Sara or Anne. Since Sara lives with Isabella for more than 20 years, I would suggest Sara is more likely. Saying the unmarried sisters Sara and Anne are sisters of Isabella in the 1851 census might be a way to avoid an embarrassing question. Similarly Sara is listed as a visitor at Isabella’s home in 1861 with no marital status given.