I assume your sister is a DVC member. She can get a discount only for family members living in same household. She would need to be present and you would all need to show IDs with her address.
I just ordered APs with the DVC discount and received the ""Annual Exchange Certificates", ie vouchers. The letter states:
To receive your new Pass, please present the attached Annual Exchange Certificates, your Disney Vacation Club Member ID Card (I'm the member who order the passes) and a valid photo ID for each adult showing they reside in the same household at any Walt Disney World Theme Park ticket window.
So, if your sister is there to show her DVC membership card and the adults all have photo IDs showing they reside in the same household as the member, this would work out.