Well, that family does seem to match, so that's good. Can't see a death reg for Rachel in the UK, perhaps she died on the voyage?
Possible birth registrations for Margaret Ann in june qtr, sept qtr and dec qtr 1867 in Neath registration district, so she could have been just 17 when she married.
Richard & Ann's marriage cert would give you the details of both their fathers. I can't see an Ann Thomas with parents David & Margaret, but I did find this possible family
1851 Garnfach, Llanedy HO107/2469 folio 333 pg 12
David Thomas head mar 40 mainer(?) & collier b. Glam, Swansea
Ann wife 41
Mary dau 11
David son 9
Sarah dau 7
Ann dau 4
Elizabeth dau 1
Elizabeth Morris visitor 20
all born Llanedy unless otherwise stated
1861 Garnfach, Loughor, Llanedy RG9/4417 folio 89 pg 2
David Thomas head mar 50 coal miner b. Glam, Swansea
Ann wife 52 b. Carms, Llandebie
David son 18 coal miner
Ann dau 14
Elizabeth dau 12
Rachel dau 9
all born Llanedy unless otherwise stated
1871 Garnfach, Llanedy RG10/5472 folio 31 pg 10
David Thomas head unm 29 collier
Sarah sister unm 27 domestic servant
Elizabeth sister mar 22 visitor
Sara Ann Evans niece 6 months visitor
all born Llanedy
suspect Elizabeth's surname should be Evans, but there is a ditto under Thomas
can't find them in 1841 though.