A search of the 1851 census on website FREEBMD identifies one Selina Williams, aged 3 years. Head of the household is Mary THOMAS, Married, an ag. lab. living in Polglase, Parish of Cury, Cornwall. Mary is living with:- her sister, Elizabeth WILLIAMS,Unmarried,aged 30 years,ag. lab. her son, John WILLIAMS, aged 11 years, ag. lab. her daughters, Susan aged 8 and Elizabeth THOMAS aged 3 years and her nieces Sarah A and Selina WILLIAMS, aged 9 years and 3 years.
A first assumption would be that Sarah and Selina are Elizabeth's daughters and, if so, it can be seen both they and Mary's oldest child, John, are using their mothers' maiden names.