Translation:I do not have an older sister but an older brother.
In my opinion, this is an awkward translation to English. Most folks don't say it like Duo. While technically orrect, conversationally it begs more: "... but an older brother" ...WHAT? ...would be nice? is no fun? I have too many? --- The sentence needs to be finished or else reworded... "... but RATHER an older brother." this word "rather" ties the end of the sentence to the subject & verb at the beginning of the sentence. In actuality, it is most common to hear this information delivered in two separate sentences: "I don't have an older sister. " followed by "I have an older brother. "
Is abi/abla necessarily "older" sibling? I think in other questions simply brother/sister was accepted. In the "translate by picking words" the choice was not available because there was no "a", just "an", but would it be OK? If not, how would you refer to a sibling not considering the relative sge?