They have done tests on those scanners, it's written up in the Unofficial guide. If you are relatively the same hand size as your sister you could probably do it.
Sharing of tickets is not permitted. The scanners are fairly accurate. Even if I use the wrong hand, the fingerscan won't register and both of my hands are the same size.
Because this discusses circumvented the official policy of Universal Studios, I am closing this thread.
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