When trying to find your brother admin password, you should know that it is not illegal. If you are in charge of his account, then it is your responsibility to ensure he doesn't have access to any personal information.
One way of doing this is by changing his password. If he does not know the new password, he will not be able to log in and see what you have hidden from him.
You mustn't share this information with anyone else because they could use it against him or use the account for their purposes.