I think I have found Edward Williams in the 1851 after a place search of the census in Llangelynin.
The household is as follows;
Mary Roberts, Head, widow, age 75, Farmer 14 acres, born Llangelynin.
Mary Roberts, Daughter, age 37, unmarried, Farmer's daughter, born Gyffin.
Edward Roberts, son, age 40, Unmarried, Farmer's Son, born Gyffin.
Edward Roberts, grandson, age 11, born Llangelynin.
On the census itself the children have "ditto" under their surnames. This is the same Mary Roberts, wife of John Roberts, as they are living in the same place as the 1841 census. This is called Pwllyrnoch (I think it is as the writing is hard to decipher.)
I wonder whether Edward was illegitimate. He was born in 1839, the same year as John Williams and Jane Roberts married.
Helig.