Hi Rosie,
Thanks for the lookup in 1841. I have the IGI  for Mary E Morris born 1834 St Thomas HWest, but are you saying she had two sisters i.e. Maria and Catherine? Can you tell me what year they were born?
You have found a Henry and Elizabeth Morris in1851 living in Llantwit.  Is that in Glam? 
I have Mary in 1851 servant living in Laws St Pembroke age 16 born St. Thomas HWest. I'm sure that's my girl, but I haven't found any sisters or brothers.  I'm now wondering if her parents have died before 1841, if so would she be in the workhouse?