My sister has a fb account .. she forgot her password ...?

[Simply RED]

If you are unable to log in to your Facebook account, click the 'Forgot your password?' link located on the Facebook homepage.
https://www.facebook.com/recover.php

Follow the steps to identify your account. If you have access to any of the email addresses listed under your account you can request that a new password be sent to one of those email addresses.

If you don't have access to any of the email addresses linked to your account click the 'No longer have access to these?' link under the list of email addresses. Follow the directions provided to answer your security question and regain access to your account.