What does your sister say about this? Her son is a minor, therefore she is responsible for his actions. I would have a very serious talk with her, and tell her if she can't rectify this situation ASAP ... like in a couple hours ... that YOU will rectify it YOUR way, and she will NOT like the results.

Call the police, file a theft report, press charges. Not only for your pet's sake, but for this boy's sake. If he gets away with stealing a monkey for profit now, what is next? Stealing a car? Robbing a store at gunpoint? Perhaps if he learns a hard lesson now, he won't have to learn a harder one in a year, and maybe pay for it with his life.

Also, call the humane society, animal control, all the vets and tell them about this. Put an ad in the local paper describing the monkey. Offer a sizable reward for it's return, no questions asked.

Don't sit on this one. Every minute you wait to do something makes it all the more likely you will never see your monkey again.