Could it be "...already" ?
Hmm, I wouldn't say so since that would be "Ydy hi'n hanner awr wedi wyth yn barod?"
It's funny because to me "... yet" means the same as "... already"
They're definitely different here. ".... yet" gives me the idea that somebody is waiting for it to be eight, whereas "... already" gives the question a tone of disbelief that it is "already eight".
Maybe regional slang? Here it would depend on the tone. If it's an actual question, it would seem like "why isn't it half past 8 already?"
Either way in Welsh they are two different sentences, since "eto" never translates to "already".
Answering "Is it eight thirty yet?" receives a wrong answer. Ond mae e yr un peth. ;-)