I see what you mean, but in this case "certo" is used to strengthen the negation, even though it's not close to non; it delivers a "you certainly can't believe that" meaning, sometimes used sarcastically ("Non è mia" "Non è certo mia" - "It's not mine" "Well it can't be mine either"). It could also have been "non è affatto nostra", to the effect of a "no way it's like that". "It's not sure that the turtle is ours" would be "Non è certo che la tartaruga sia nostra".
Ciao Formica: could you please direct us, the people leaning here, to the rule which indicates that an adjective is use in Italian to strengthen a negation? As far as I know, it is an adverb that modifies the verb and not and adjective. Here the verb is è follow by an adjective certo.
"Non è certo che la tartaruga sia nostra" (It would require the subjunctive tense of essere - sia. If you don't know what that means, you'll get to it later. Sentences with an inflection of doubt use a different tense in Italian, and in proper English grammar for that matter)
It's very unusual word order for English. I can't even figure out how to write what it means. Something like "it's not certain that...", as in "Non é certo...". But this means "it's definitely not ours." (I came here because i didn't understsnd the Italuan word order. This is how i understand the meaning of the Italian.) We have to get used to the fact that different languages use different word order, and adverb placement is one of the really difficult things for English learners. (I used to be a teacher of English for foreigners.)
By Collins, certo can be an adjective or a an adverb, as an adjective it changes by the gender of the subject, but as an adverb it doesn't. Here, as f.formica explained - it is an adverb - therefore it can only be certo, no matter what is the gender.
There is also an adverb 'certamente', but I don't know if the two are interchangeable fully, or depend by the context.