Yes, you are allowed to upgrade others not flying with you, using miles from your account. What you would need to do is call up UA Reservations and give the agent the itinerary info for your sister, and your own MP information. Since you are not flying with her, you would need to visit an airport ticket counter and authorize the use of your miles for her trip by signing an auditor's coupon.

Furthermore, to use upgrades, follow the following rules:

http://www.united.com/page/article/0,6722,1150,00.html


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