the 1851 census of Glamorgan has 2 John & Rachel Webber families. One Rachel is 31, born Llanegwad & is in Cadoxton, the other Rachel is 38 & is in Merthyr and would appear to be the one from the marriage cert you have.
1841 Gellideg Bridge, Merthyr Tydfil HO107/1415/4 folio 30 pg 22
John Webber 30 Beer House Kr
Rachael 28
John 8
Jane 1
Esther Jones 17 Female servant
all born in county except Esther. The gap between John & Jane suggests this was a second marriage for John.
1851 census for this family shows Rachel's place of birth as Merthyr Tydfil.
the other family in Cadoxton
1851 Cadoxton HO107/2464 folio 321 pg 4
John Webber head mar 39 Railway labourer b. Glam, Llansamlet
Rachel wife 31 b. Glam, Llanegwad
Ann dau 10 b. Carms, St Peter
Thos. son 7 b. St Peter
John son 4 b. St Peter
Elizth dau 2 b. Cadoxton
Thos. Morgan visitor 19 Railway labourer b. St Peter
James Rainols? 50 Railway Labourer b. Neath
This family should be in Carmarthen in 1841 given the place of birth of Ann.
Which family are you researching? Which child are you descended from?

red post - I'll post this to show 2 families.