Wow, I'm sorry that happened to your sister. Probably the best thing for her to do is figure out her legal options. Would you saying how much money she lost, and what country or region she's in? That would tell us what sort of laws there are were she lives, what sort of laws the person who tricked her may have broken.

As very general advice, I think most legal systems have something along the lines of criminal law and civil law. In English-speaking countries at least, criminal law is about the government suing and punishing people, and civil law is about people and businesses suing and getting restitution. If your sister sues under civil law, that may take more time and money than it's worth. If she reports it to the police, the downside is that providing information and perhaps being called as a witness could take up a lot of her time.