I had a question: Since I'm studying Dutch Law and we are doing criminal law at the moment something came up in my mind. In the Dutch Law there's an article that says that it's not obliged to inform police/justice (when they ask about it) about illegal activities of family, like father, brother etc.....you can simply keep your mouth shut. You don't get punished for that...

Is there a sort of same article in American Law?

And secondly when there is an article like that then it's very logical that organized crime families only have members who really are family (fathers sons uncles etc...), thus the old Sicilian structures. You have a (little) stronger position then when you have outsiders in your Crime Organization.

I'm not certain, but I don't think that there's anything in American law that requires that we report (without being asked) any illegal activity that we may be aware of. Other than a moral requirement, that is. But not a legal one

But if asked by the police, it can be considered obstruction of justice, a criminal offense, if we lie.

However, if called upon to testify in a court of law, then naturally we have a legal obligation to tell the truth.

An American cannot, however, be forced to testify against their husband or wife.

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.

Okay thanks for the answers...

So if I'm correct an American is forced to give explanation (in court) about illegal activities of (for example) your cousin (as Castellano-Gambino)...but not wife/husband?

Because in the Netherlands it's the same rule but then to the 3th grade. So uncle/cousin is still protected.

Yes, but only under certain circumstances. One more detail:
Suppose a prosecutor hauls you before a grand jury investigating the criminal activities of your father, brother, cousin, etc. You could "plead the Fifth": claim that if you answered questions about your father's criminal activities, your answers would incriminate you because you were a partner, witness, etc. The judge then could offer you "immunity to testify": that is, he could say to you that you will not be prosecuted for any crimes you admit to while testifying, in return for answering all questions about your father truthfully. If you still refused, you could be held in contempt of court. You see, the Fifth Amendment protects you against incriminating yourself. But if a judge guarantees that you won't be prosecuted if you testify, the law assumes that you no longer need that protection--hence, you can be held in contempt of court if you refuse.

it's clear now, thanks