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Double check the email address that your sister used to send the funds and make sure that there were no spelling errors or misplaced dots etc.
Ask her if its showing as pending or unclaimed at all and if there is a cancel option.
Have you received funds before and is your email address verified?
Has she sent it to the email address that is linked to your paypal account and not another one if you have one.
Lastly is your sister in Canada as well and if not what currency is she sending, and have you added it as a receiving currency to your paypal account?