Welfare fraud is a felony so this is a serious matter. You could potentially receive prison time if convicted.
If you signed the application, then just like tax returns, you can be held accountable for the contents even if your wife actually filled in the documents. Your wife could also be held accountable (and they might seek your assistance in prosecuting her.)
You should not talk with the investigators again without the assistance of a lawyer. They are not trying to help you, but trying to use your words to assist them in prosecuting you (and probably your wife.)