You don't say what the nature of the civil suit is. The information in tax returns you sign and file, whether or not joint, is discoverable if it is relevant to the suit. Whether or not it is relevant is something a lawyer who is familiar with the suit would have to advise you on. The determination of relevance ultimately is made by the judge in the case. If you are concerned about third parties seeing your wife's income (as opposed to the other side in the case), you may be able to obtain a protective order from the court prohibiting further disclosure.
This response contains general information only and is not legal advice on which you may rely. It does not establish an attorney-client relationship.
Spousal privilege typically applies to communications made between spouses in private. Since a joint tax return is seen by a third party (the IRS), it is most likely not protected by spousal privilege. A lawyer in your state can advise you on whether or not the tax return is discoverable and if there's anything that can be done to exclude it.
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They are discoverable unless you could get a protective order. Here is why: parties may obtain discovery regarding any matter, not privileged, that is relevant to the claim or defense of any party, including the existence, description, nature, custody, condition and location of any books, documents, or other tangible things and the identity and location of persons having knowledge of any discoverable matter. For good cause, the court may order discovery of any matter relevant to the subject matter involved in the action. Relevant information need not be admissible at the trial if the discovery appears reasonably calculated to lead to the discovery of admissible evidence. Rule 26, C.R.C.P.
The information provided in this answer does not create an attorney-client relationship. If you are interested in a free consultation with Mr. Bryans, call The Bryans Law Office at (303) 832-2930.