"Our aunt is the most expert cook in the family."
Translation:Nia onklino estas la plej sperta kuiristo en la familio.
Since we're comparing to the cooks in the family, then it's obvious, we're not talking about licensed, trained cooks, but just an ability of preparing food for oneself. So it should most certainly not be kuiristo but kuiranto (or kuirulo, maybe), since -ist- means a profession, occupation or a job position.
If you're a professional musician, who works in a choir, you're a ĥoristo, but if singing in a choir is just your hobby, your only a ĥorano. If you play football for fun, on the weekends with your friends, you're a piedpilkanto, but you can call yourself a piedpilkisto only if that's your job to compete on a field with other footballers.
I tried to be smart and entered<pre>
Nia onklino estas la spertega kuiristo en la familio</pre>
or maybe in addition to what I'm reading below I could have entered<pre>
Nia onklino estas spertega kuiranto en la familio</pre>
It (the first one) was not accepted and I'm not sure if this is correct. Can one of you validate this for me? I think maybe the la in front of spertega needs to go too.
As a word order, it's perfectly fine. But it doesn't work particularly well in this sentence, since it can convey a slightly different meaning. It all depends on the context and even on one's intonation in this sentence, but by default this change of the order between the noun and it's description would be understood differently, by analysing what's being made definite by the article la.
la plej sperta kuiristo — would be “the professional cook, who is the most experienced in his profession”
la kuiristo plej sperta — would be “the professional cook, who is one of the most experienced in this profession”