Forgot to add, John and Ann are alive in 1861 at 1, Plantation Rd, Merthyr.
John is a Moulder by occupation. And in 1851 is living in Aberdare with Evan Hopkins and wife Margaret. Stated as Brother in law. But when I search for the Evan Hopkins marriage I find that Margarets father is John who is also a Moulder. This marriage was in 1845 in the parish church of Aberdare.
For info. - no relationships given on 1841 census this appear to be
Margaret & John LEWIS with their parents and siblings living in Aberdare in 1841
Piece 1420 Folio 63 Page number 22
John LEWIS 45
occ MoulderMargaret 45
JOHN 15
Alexander 15
MARGARET 15
Jane 13
William 8
Thomas 4
David 2
1851 c Piece 2460 Folio 497 Page number 44
John LEWIS 57 Head Merthyr occ ?
Margeret Lewis 56 Wife Aberdare
William Lewis 18 Son Aberdare, blacksmith
Thomas Lewis 14 Son Aberdare
David Lewis 12 Son Aberdare
Margeret Thomas 27 Servant
Ann
HOPKIN 5
Granddaughter Penderyn, Breconshire
Not found baptism for their son John

there are these baptisms
Alexander LEWIS
Baptism 11 Jul 1824 Merthyr Tydfil, Glamorganshire, Wales
Denomination Wesleyan
Father John
occ moulder Mother Margaret abode Parish of Aberdare
England & Wales, Non-Conformist and Non-Parochial Registers, 1567-1936
Margaret LEWIS
Baptism 26 Nov 1826 Merthyr Tydfil, Glamorganshire, Wales
Denomination Wesleyan
Father John occ
moulder Mother Margaret
Jane Lewis Birth 21 Jan 1829
Baptism 3 Feb 1829 Merthyr Tydfil, Glamorgan, Wales
Denomination Wesleyan
Father
John Lewis of Hirwaun
Mother
Margeret dau of William & Margaret
BEVANWilliam Lewis Birth 11 Feb 1833
Baptism 4 Jun 1833 Merthyr Tydfil, Glamorgan, Wales
Denomination Wesleyan
Father
John Lewis of Hirwaun,
moulderMother
Margaret, dau of William & Margaret
BEVANEngland & Wales, Non-Conformist and Non-Parochial Registers, 1567-1936
LEWIS, DAVID
mmn BEVAN GRO Reference: 1839 J Quarter in MERTHYR TYDFIL UNION Volume 26 Page 365
Possible marriage not sure if surname mistranscribed

could read Bevan
John LewisMarriage 24 Jan 1820 Aberdare St John, Glamorganshire, Wales
Residence Place Penderyn
Spouse
Margaret BowenGlamorgan, Wales, Anglican Baptisms, Marriages and Burials, 1570-1994