I still think your best bet is the family baptised in Bethlehem Chapel, Pwll Trap which is only about 2 miles from Llanddowror. Perhaps they lived between Llanddowror and St Clears when Martha was born. I can't find James and Ann in the 1851 so they could have died by then.
Just to confuse you there is a record of a Mary Lewis baptised in Llanddowror Church, 29- 8- 1824, to James Lewis and Elizabeth. I think this couple are later found in LLanfihangel Abercowin.
There are several Lewis families around Llanddowror, including a Benjamin Lewis and wife Hannah who also baptised in Bethlehem chapel.
What names did Martha use for her children? There is often a clue to family members here - parents, siblings, uncles and aunts.
I think you can discard the Martha living with Griffith Lewis in St Peters. She is still there in 1851 and described as his daughter.
Sorry not to be of more help.
R.