I don't think you'll find them as a family in 1871 as William & Sarah Ann marry in 1872.
On checking the GRO site for maiden names, there's a possible birth reg for Elizabeth Ann dec qtr 1876 Pontypool vol 11a pg 123 mmn Roberts, likewise a possible for William dec qtr 1875 Pontypool vol 11a pg 142 mmn Roberts. No obvious birth reg for Mathew, but the possible for Harriet march qtr 1865 vol 11a pg 124 has mmn Williams.
In Llanhilleth parish church Nov 2 1872 William Jones, full age, widower, labourer of Llanhilleth, father Joseph, labourer, married Sarah Ann Roberts, full age, spinster of Llanhilleth, father John, haulier. Both marked, witnesses William Williams and John Arunell(?)
1871 Pontnewynydd RG10/3561 folio 4 pg 1
Mary Roberts head widow 54 b. Glascoed
John Williams son 26 labourer b. Glascoed
Sarah A dau 19 b. Glascoed
Mary dau 16 domestic servant b. Glascoed
Louisa dau 11 b. Trevethin
Mathew grandson 1 month b. Pontnewynydd
So Mathew would appear to be Sarah Ann's son. Birth reg as Roberts march qtr 1871 Pontypool vol 11a pg 147 no mmn.
Possible first marriage 9 Feb 1863 in Trevethin William Jones 28 labourer of Trevethin, father Joseph, labourer, married Mary Ann Williams 20 of Trevethin, father William, woodward, both marked, witnesses Joseph Wayman & Henry Williams.
Possible death reg
Mary Ann Jones 25 jun qtr 1868 Pontypool vol 11a pg 86