It is a tricky one. The entry is for a Mary Williams widow age 78, with her are daughters Martha 28 and Sarah 30, both born Llanelly. She was married to a William Williams born abt 1781 Kidwelly. I can’t find her in 1851, so nothing to compare with. In 1851 she is not with her husband, who is at No. 6 Caemain, Llanelly with daughters Margaret age 30, Martha age 19 and nephew William Williams age 10 and Neice Mary Jane Rees age 10m all born Llanelly. In 1841 William, Mary and their children are again at Caemain. In 1841 it says “Y” for Mary to “if born in county”, this obviously contradicts the 1861 entry but the entry in 1861 specifically states ‘xxx’ Glamorganshire, so I think the “Y” in 1841 is probably an error?