Translation:My older sister is good at playing piano.
That's BS, sorry. The Japanese language habitually specifies the relative age of siblings, whereas the English language usually does not.
The act of translation always has to take cultural differences into account, otherwise why even bother? Google will be better with literal translations in no time.
Gabriel, they didn't require the specification of "older" for 兄 (あに), so it's inconsistent to require it now for 姉 (あね).
Since this test is for comprehension and learning checks, I agree that we should be answering specifically (unlike normal English), but inconsistencies in the answer keys are unforgivable, and trick users into making "mistakes."