No luck finding Thomas/Hannah together in '51.
FreeBMD has the marriage of Thomas Griffiths, Sep 1851, Tregaron, 27 29. On the same page is a Hannah Lewis.
Might this be your Thomas & Hannah - or have you already established they were together at the time of the '51 Census?
Not sure about Jane and Catherine being Hannah's sisters ie
1851 Census
David Lewis, Head, 59, Weaver, St Peters Carm
Mary Anne Lewis, Wife, 36, Dressmaker, Llanddewibrefi
Mary Lewis, Dau, 13, App Dressmaker, "
Hannah Lewis, Dau, 10 , "
David Lewis, Son, 7, "
Catherine Lewis, Dau, 5 "
Jane Lewis, Dau, 3, "
Micah Lewis, Son, 11mths "
Charles James, Lodger, Widower, Chelsea Pensioner, Llandilo
1851 Census HO107 2487 274 14
Micah Lewis, head, 59, Shopman, Town, Carms
Elizabeth Lewis, Wife, 58, Town Carms
Hannah Lewis, Dau, 26, llanddewibrefi
Richard Joseph, Serv, 37, Llanfarddydegau
William Joseph, G/Son, 11, Llanddewibrefi
John Joseph, G/Son, 8, "
Margaret Jones, Serv, 28, "
Hannah, age 26, fits the Hannah married to Thomas in 1861. Hannah, aged 10, living with Jane and Catherine in 1851 would not appear to be the Hannah married to Thomas and living with Jane and Catherine in 1851..
Any thoughts?