Try to find all the records by looking into the payment methods (credit card history) which your sister paid with to have the services performed... And then contact the businesses to reprint / email the invoices to you.
Don't give up, and keep in touch with the manufacturer regarding this. The manufacturer is the one responsible for covering the repair, so make sure you consistently follow-up with the manufacturer's customer relations department.
If you truly feel you should be covered, remember to NOT give up regarding getting covered by warranty... Good luck!
