Maybe she decided to transfer to a school with a Greek system after being told that pledging as a med student wasn't possible.
That's a stupid reason to transfer, if it's the case.
OP -- you need to worry about your education, not becoming a member of XYZ sorority. If you changed your mind about where to attend school, I hope it was for a good reason.
Juniors and seniors who are full-time at any school are certainly allowed to. However, whether they actually have a good chance at receiving a bid depends on the school you go to.
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"Remember that apathy has no place in our Sorority." - Kelly Jo Karnes, Pi