That's actually a matter where some interesting linguistic judgment might be in order.

The "ain't" part is grammatical in some dialects of English, though, as far as I know, not in any form of standard English that is officially recognized anywhere. But the wrong cases for cetera and pares are not grammatical in any form of Latin that has ever been spoken or written anywhere.

On the whole, I'd say that "ain't" is less bad, since in the dialects in which it is grammatical, it has the same form for both singular and plural. Therefore, at least it respects the number agreement with the Latin plural cetera, whereas "is" commits an additional offense by violating that agreement.