No relationships given on 1841 census if we presume Sarah is his daughter and named after her mother there is this possible marriage
David Williams
Marriage 17 Nov 1806 Merthyr Tydfil, Glamorganshire, Wales
Spouse: Sarah Richard
married after banns, both single both made their mark
(a mistake I think with surname for David written as Richard)
In presence of William Jones & William Williams
Edit to add still looking for a baptism for Sarah & Jacob
found Catharine’s and her marriage
Catharine Williams Birth 1 Apr 1823
Baptism Date: 27 Apr 1823 Merthyr, Glamorganshire, Wales
Denomination: Presbyterian
Father: David Williams Mother: Sarah
England & Wales, Non-Conformist and Non-Parochial Registers, 1567-1936
Marriage 9 Nov 1846 Merthyr Tydfil, Glamorganshire, Wales
Catherine Williams f/a father David Williams, miner
William Griffiths, miner father George, miner
Catherine made her mark
Witness W Rowland & Jacob Williams
Marriage 10 Mar 1847 Merthyr Tydfil, Glamorganshire, Wales
Jacob Williams f/a, miner David Williams, miner
Gwenllian Howells f/a father Thomas, coal agent
all signed
Witnesses W Rowland & John Howells
Glamorganshire, Wales, Anglican Baptisms, Marriages and Burials, 1570-1994
edit to add source baptism & marriages