Is this your David and Mary Evans, married Jun Qtr 1868, Swansea?
1871 Census RG10 5445 10 1481
David Evans, Head, 31, House Joiner, b. Llandilo, Carm.
Mary Evans, Wife, 35, b.Llangyfelach
Mary J Hughes, step dau, 14, b. llangyfelach
John Hughes, step son, 12, b. Llangyfelach
Margaret Evans**, dau, 2, b. Llangyfelach
Address Brynyfrydd, Llangyfelach
** GRO birth registration, Margaret Ann Evans, Jun Qtr, 1869, mother's maiden name Roberts
1881 RG11 5352 26 46
David Evans, Head, 41, Carpenter & Grocer, b. Llanddebia, Carm.
Mary Evans, Wife, 45, b. Llangyfelach
Margaret Evans, dau, 12, b. Llangyfelach
Elizabeth Evans**, dau, 8, b. Llangyfelach
Sarah Evans**, dau, 10, b. Llangyfelach
John Howell, son in law, 24, Potter in spelter works, b. Llangyfelach
Mary Jane Howell, dau, 24, wife (to potter in spelter works), b. Llangyfelach
Margaret Edith Howell, dau, 2, b. llangyfelach
Address - 15 Pentreharn Hill
I can find GRO birth registrations for Elizabeth (Jun Qtr 1873) and Sarah (Jun Qtr 1871) - mother Roberts
No sign of an Ellen Evans, born about 1878, either on GRO or the above '81 Census.
On the 1901 Census Ellen Williams, wife of Morgan, has place of birth "Cockett".
If you have her marriage cert what was Ellen's father's name and occupation?