I could not find any with a Jack and Sarah with a mary ann in Carmathenshire,
but here is one that I found with a Sarah and Mary A Thomas, if you have anymore infor to help I could look further.
Mary A Thomas
Age in 1901: 13
Estimated Birth Year: abt 1888
Relation: Daughter
Father's Name: David
Mother's Name: Frances
Gender: Female
Where born: Llanelly, Carmarthenshire, Wales
Civil Parish: Llanelly
Ecclesiastical parish: Llanelly St Elli
Town: Llanelly
County/Island: Carmarthenshire
Country: Wales
Registration district: Llanelly
Sub registration district: Llanelly
ED, institution, or vessel: 22
Household Members: Name Age
David Thomas 43
David Thomas 7
Frances Thomas 42
Martha Thomas 5
Mary A Thomas 13
Oliver Thomas 9
Sarah Thomas 12
Willie Thomas 10