Ok in outlook click on tools- then click on accounts- then view accounts and you will see your account and your moms. highlight your account and on the right hand side click remove.
Outlook can only download e-mails for accounts that are setup in it. So if outlook is downloading your e-mail. Your account is setup in it. So you will have to remove the account from outlook and then only your moms account will be there so outlook will only get her e-mail