Once a subpoena is issued it compels the person served to comply with whatever is requested it. Accordingly since you have been served, you are under a legal obligationto testify at your wife's trial.  Failure to comply with a subpoena is a criminal matter for which there are penalties, including fines and jail time.  Additionally, a witness can only "plead the 5th" (i.e. invoke their “privilege against self-incrimination”) if their testimony would implicate them in some way in the crime.  So unless your testimony can get you personally in trouble, you have to testify Note:  Even if you could plead the 5th, as a witness you would still have to apper in court to do it.