You won't have the possibility to remove that message unless the phone is rooted or jailbreaked, because there are apps that have the possibility to clear Snapchat logs, but removing an individual message is still not possible since those apps will remove all there is to log keeping in Snapchat. So, unless you remove the software, there is no way to get rid of that message. Simply uninstall it and then install it back, or deauthenticate your brother's account and then authenticate again.
