You are very lucky as she appears on the 1901 Census

Birth Reg;
Gwyneth Eira SEABOURNE, December 1899, Pontypridd, 11a 498
1901 Census
2 Bethesda Street,
Ystraddyfodwg, Glamorgan, Wales
RG13/5002/58/2
John SEABOURNE, 32, Head, Mar, Stationary Engine Driver/Colliery Surface, born Glamorgan, Hafod
Mary, 28, Wife, Mar, born Monmouth, Tregear
Mabel B, 8, Daug
Bertie, 7, Son
Lilian, 4, Daug
GWYNETH E, 1, Daug
Eunice V , 3mths, Daug
All children born Hafod, Glamorgan
Gwyneths Birth Cert will give her Mothers maiden name and then you can look for a Marriage.
Trish
