I am using Ancestry.uk which is a pay site.
1861 census Mangotsfield Gloucester
BRYANT
George, 44, carpenter employing 2 men
Mary, 43
Henry, 21, carpenter
John, 19
James, 15
Robert,11
Elizth, 1
All born Mangotsfield Gloucester.
Another Robert in Gloucester with sister Elizabeth aged 6 on 1861 census. Parents Mark and Elizabeth.
1871 census, parents Mark & Elizabeth has an Elizabeth aged 16.
I am working on the assumption that your Robert is the one with parents George and Mary. Hope I am right.
1871 census Mangotsfield
BRYANT
George, 52, carpenter
Mary, 50
Robert, 20, carpenter and joiner.
Elizabeth Bryant is a fairly common name in the area and they are several born re 1861 or so.
As she isn't on 1871 census, is there a possibility that she died? If she was still alive she would only be 11 and would still be at home.
One of Robert's children is named Bessie.