plaw got it absolutely right, MM. No American is required to answer questions posed by police on any subject, about anyone. But, under some conditions, a person who is suspected of being an accomplice of a criminal can be called before a grand jury to give evidence. Let me explain:
When someone is arrested in the US, he is brought before a judge within 24 hours of arrest, and is "arraigned": the judge hears the charge, sets a tentatived trial date, and determines the amount of bail (if any). Then the prosecutor gathers evidence against the accused and brings it before a "Grand Jury." This is a group of 24 citizens that meets continuously to sift evidence in many criminal cases, not just one. Their job is to determine if the evidence against the defendant is sufficient to indict him for the crime (known as "returning a True Bill"). This isn't a trial--it's a determination if the case really should go to trial (the Grand Jury almost always agrees to indict, and leaves it to the trial jury to determine innocence or guilt).
Now to the point: the prosecutor can ask the judge to subpoena witnesses against the accused to testify before the Grand Jury. Although the Fifth Amendment to the US Constitution specifically prohibits self-incrimination (you don't have to testify against yourself), you can be compelled to give evidence against someone before a Grand Jury--not your spouse, as plaw said, but perhaps a family member. If you refuse, the judge can hold you in "contempt of court," and put you in jail for as long as the Grand Jury is in session, which could be months.
In a famous example in the US: More than 50 gangsters were arrested at the aborted Apalachin, NY gangster convention of 1957. Many were hauled before a Grand Jury and ordered to give evidence. Paul Castellano was arrested at Apalachin. He was asked to give evidence before a Grand Jury against other gangsters--including his cousin and brother-in-law, Carlo Gambino. He refused and was held in jail for months for contempt of court. He was released after a higher court ruled that that particular Grand Jury proceeding violated the US Constitution. His willingness to go to jail for contempt of court rather than give evidence against others (including his cousin) gave Castellano the reputation of being a "stand-up guy" and helped propel his Mob career.