Your spouse may have started out as an Au Pair when she entered the country 2+ years ago, but Au Pairs are not allowed to remain in the country for more than one year. That takes her out of the category.
Here is an IRS link to information about Au Pairs
She meets the substantial presence test and you don't need to sign any declaration for her.
Here is a TurboTax article about resident and nonresident aliens.
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