Are keyloggers legal?Watch
Probably not the best tool to ensure that your brother's internet use remains child friendly.
Depending on your brother's age, your parents may want to consider installing a kid friendly browser and parental controls .
These links might be helpful:
Main function of keyloggers is recording all keystrokes made on a computer, as also all other specific information like web searching, using of text documents, email, chatting, social media activities and so on. They work in a hidden mode and some of them are really difficult for detecting and blocking. Because of that they're a very popular solution for parental control and in companies for controlling their employees work time.
In your case using of keyloggers will be definetely ok but install them only on your own gadgets or the ones which your brother uses. It's totally legal for parents or other relatives to use keylogger software https://www.refog.com/ for their underage children in order to monitor and control their activities and protecting them from viewing an inappropriate content.