You will need to start with a tax return transcript and an account transcript, to see if a refund was paid. Assuming you don't have online access to your sister's account, you will have to file the request by mail. I would probably attach a copy of form 56 to form 4506T.
https://www.irs.gov/forms-pubs/about-form-4506-t
Then if you find a check was issued, you will need to initiate a Trace. Again, you may not be able to use the phone system and may have to file a paper form.
https://www.irs.gov/faqs/irs-procedures/refund-inquiries/refund-inquiries-0
