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Normally, INS just requires a copy to be filed. If you intend to submit affidavits, in that case you need a non availabilty certificate from the municipality of the town your wife was born in.
Since she has a copy, it means her BC is available. I would suggest you should try to get her original BC.
Its a very high probability that you\'ll never need original. But who can fathom the ways of INS. Just in case they ask to see original!