This question already has an answer here:
Generally speaking, doing know sins for a good reason is allowed?
You suspect your brother is doing something wrong with his free time on the internet and you want to protect him, so, when he is out of the house you go to his computer and browse the history.
Viewing personal information like the websites someone visited without his approval is clearly a sin. But, if you do this in order to protect him from talking to bad people online, is it allowed?