Yes, you should send the notice of intent to defend to the court and also to the attorney who is acting on behalf of the plaintiff. Be careful of what you write in your notice of intent to defend as anything you write can be used against you during the case.
If you were listed as a joint borrower on the credit card account when it was opened, you are probably liable for the debt. You need to find out if the credit card company included you as a joint borrower (which is highly likely). Most credit card companies wouldn't use your credit history to open an account for your wife without including you as a joint borrower.