Most courts hold that the incredibly fragile attorney-client privilege can be waived by disclosure even to family members (such as Martha Stewart’s disclosure to her own daughter). The separate "spousal privilege" recognized in many states complicates the analysis with those family members.
In IsoNova Technologies LLC v. Rettig, the court noted that "[s]ome courts have . . . held that the presence of a spouse on an attorney-client communication destroys the attorney-client privilege unless the spouse's presence was necessary for the lawyer to effectuate the representation" (which is the standard for all client agents and normally fails). Case No. 20-CV-71-CJW-KEM, 2023 U.S. Dist. LEXIS 94454, at *11 (N.D. Iowa May 31, 2023). The court referred to Iowa's spousal privilege statute as taking the more generous contrary view. Noting that the Iowa statute protected the defendant's disclosure of privileged communication to his wife, the court explained that "[i]t does not make sense then that by including his spouse from the start, the emails lose their privileged nature." Id.
Lawyers should still warn their married clients to be very careful — states recognizing a spousal privilege may place that protection in different places (statutes, rule, etc.) and take widely varying approaches (applying just in criminal cases, etc.). The good news is that work product protection almost always survives disclosure to friendly third parties like a spouse.