There are two possible births for Sarah in the Builth area, you could try contact the relevant reg office or the GRO and quote parents as ref to obtain right cert,
You would then have Elizabeths maiden name to help trace her on census, also to confirm if they did marry.
I have tried to locate Elizabeth prior 1881 & William/Elizabeth 1881 but no luck. they could be mistranscribed but with such common first names it is needle in a haystack.
There were others but these are the only ones reg in Builth, worth trying if your at a brick wall.
Births Dec 1881 HUGHES Sarah Ann Builth 11b 80
Births Mar 1882 Hughes Sarah Builth 11b 88
hope this helps, good luck
Cas