All my music has been deleted off my phone, and I don't know how to get it back!
If your brother deleted the songs themselves they are gone unless your computer is backed up. 

If your brother just deleted the playlist you need to recreate the playlist and then re-sync it.  Here is a how to article on how to do that. - http://www.ehow.com/how_2214496_sync-playlists-iphone.html