But..
If I have the right family in 1851 and 1861, living in St Dogmells, with oldest daughter Mary, William John is a fisherman not a collier.
In 1841 there is a William John (fisherman) age 25 (rounded) with Mary age 60 , Pheoby 20, Maria 15 - presumed mother and sisters. Mary's occupation says Ind(ependant). Normally for a married woman occupation is left blank, so I suspect she has been widowed.