Welcome to FT! I second the refundable-ticket suggestion. It works.
And following it is not as hard as you might think. If your sister is less experienced than you at dealing with airlines, then you can buy the ticket in her name, she can check in at a kiosk, and you can do the refund process together either phone or online once she is with you. The hardest part depending on the airline may be that you have to "un check in" her prior to issuing the refund, and that's not very hard. (If all else fails, go to a gate agent.)
I have done this many times, both as the visitor and the traveler.