When Mary Curtis was married in 1838 her father is given as John Curtis, an Excise Officer. Censuses say she was born in Ashwell, one giving it as Herefordship but it was actually Hertfordshire. Her birth was registered in 1837 by Wolston Calvinistic Baptist Church when her mother is given as Sarah Curtis, formerly Oliver.

Therefore the fact that Sarah Oliver is the mother of Mary Curtis seems fairly certain.

It was then possible to locate John and Sarah in various censuses with another daughter, Unna Ann whose birth was recorded at the same time as Mary's.
From this it is known that Sarah was born about 1787 in Hoo, Kent.

There is a baptism entry for Sarah Oliver, 30 September 1787 at Saint Werburg Hoo, Kent. This is a different village to and a few miles south of Saint Mary Hoo. This establishes here parents as Thomas and Mary Oliver.

Note that the  Phillips family used to have a Dictionary which was inscribed  Mary Oliver MDCCLVIII (1758).