Deleted /usr/local/bin and /usr/bin on OS X
/usr/bin is where most of the Terminal commands are stored (e.g. clear). When your brother deletes everything in /usr/bin, he basically deletes most of the executable files that the Terminal and the system use. So the only way to get them back is to reinstall the OS. To do that, boot into recovery mode by holding cmd+r during startup, then after it boots up, select Reinstall OS X and follow the steps to reinstall OS X. It will not erase all of the files that you have. It only writes some system files (all the files in /usr/bin for example) back to where they belong.