That is true in federal court. If she wants to, she can testify. In my state there is an absolute bar. If I am a defendant in a criminal trial, my wife can't take the stand unless the crime was against her. She could also divorce me. Nucky got the case bumped to federal court so she can testify if she so desires.

There are two types of marital privilege: privileged communications and the testimony privilege. Privileged communications are anything discussed between spouses during the marriage while the testimony privilege prevents a spouse from testifying against the other about anything. It could allow her to not testify.