Is looking through someone's personal financial information illegal?

The main issue is, why were they in your apartment in the first place? Second, do you share that apartment with your boyfriend ? if so, are you both on the lease or rental agreement? (That affects whether or not he has the authority to allow them to come into the apartment without your specific consent.) Next, are any of your documents missing or damaged? Unless the parents had specific permission from an actual tenant to enter the premises, what they have done is a wrongful trespass and an invasion of your privacy. While a real solution to this matter probably depends upon your relationship with your boyfriend, if you have any proof that his parents entered that apartment without your permission and went through your stuff, that would be grounds for a legal action against them. If they took any of your stuff and used it to cause you any harm, they could also face criminal prosecution.