System enters wrong default name in Mail

Hi everyone,

I've noticed that my computer has the wrong default name when I add new mail accounts, both in mail.app and System Preferences > Mail, Contacts, and Calendars. When I click to add a new account in either application, the name field is automatically filled in with my brother's name (who has never touched my computer, is not on my local network, nor have I created any accounts for him). In System Preferences, my four Gmail accounts appear with his name listed so I deleted and re-added the accounts and they still keep defaulting to his name. If I click to add a Microsoft Outlook account, the wrong name also autofills which is why I suspect this is a system-wide issue (at least with email, but not exclusive to Gmail). My accounts in mail.app are under my name as well as the Users & Groups in Sys Prefs. Does anyone know what might be the issue? Thanks in advance for your help.