login with your "normal" admin accoun> then right-click on my computer> go to "manage"> go to local users&groups> go to users> right-click on "administrator" and click "change password" and set a new password.
If you have any encrypted files under that account, they will not be accessible. But, this will get you into the account.
Dont let your younger bro have admin rights, and dont leave the computer logged in unattended unless you lock it.