There is no way of knowing if the 3 grandchildren shown on the 1851 census were all sisters, they could all have had different parents. The following are possible clues, but nothing for certain has been found.
Frances Cornwell, baptised Hadlow Down, Sussex as Fanny Jenner Cornwell, 6 Jan 1850, mother Sarah Cornwell, a servant at Coles Hall.
Marriage of Frances Cornwell, 19th October 1873, age 24, Mayfield to Samuel Barton, no father's name shown on the marriage certificate.
There is an entry for this family on [http://www.theweald.org] which may help.
Eliza Cornwell, baptised Waldron, Sussex, 9 Jun 1844, daughter of Sarah Cornwell, a spinster. Marriage 23 November 1867, Rotherfield to James Bonds, father's name Thomas Cornwell, deceased.
So it is just possible that all three girls were the offspring of Sarah Cornwell, and at their marriage Sarah Ann and Eliza gave their grandfathers name as their father, rather than admit to being illegitimate.
I'm sorry that I can't be more specific with this, good luck with your research.
(Sources used, Familysearch, Findmypast, Sussex Family History Group website, http://www.theweald.org/)