...Yes, I may have written an honors thesis on this topic....
Doctor Bomb said:
Sort of.
At sea, in the open ocean, Admiralty Law applies. Basically, the ship's captain is the law of the land, and anything he/she approves of is sort of legal. Of course, once the ship returns to port, it falls under the local jurisdiction, and complaints may be taken to the constable/port authority/high priest/city watch, and they may or may not take action. While many countries outlaw piracy, it continues to this day in the waters around the Phillipines, Malaysia, and Indonesia. And of course, there is always the "legal piracy" known as privateering, where pirates fly the flag of a patron nation to sack/steal/sink the vessels of rival nations.
Privateering is no longer legal, and is one of the few international crimes which allow international jurisdiction in bringing the perpetrators to justice. In essence, pirates and privateers are safe nowhwere. Anywhere they're caught, they either 1) must be tried in a public court, or 2) extradited to a country that will try them.
Even on open waters (or the "high seas," in nautical parlance) piracy is illegal because it is governed by international law and treaty. Thus, the laws of a particular nation don't matter - since there is a structure of law that supercedes them, in this case at least.
Obviously, there isn't a world government (no, the UN doesn't count) that can enforce these laws. Essentially, it's only the force of international pressure that ensure these laws are followed, and enforced. In places where the rule of law is perhaps a little weaker than others, however, this kind of international crime (now largely being considered Terrorism, as opposed to organized crime) can continue. The western mediterranean, the persian gulf, and parts of the pacific ocean are generally where such activities take place.
In short, tell your sister that she may need to reconsider her career options. If, for example, she were to prey upon a cargo ship in international waters, and be happened upon by part of - say - the British navy, they would probably board her vessel or fire upon it. If she survived, she would have no legal recourse, and would probably be imprisoned for the remainder of her natural life. Even in these modern days, people haven't forgotten the kinds of trouble Piracy (and the break down of international law it represents) can cause.