I think your sister will be fine, but I won't be surprised if ACL cancels at least some of the shows. I also won't be surprised if some shows still happen. If not outside, then inside other clubs.
I think she should still come. It might rain, but Austin's a great town regardless, and I bet she'd still have a good time.
I would be aware of one thing, Jackson, MS airport didn't reopen immediately and restricted outgoing flights so the same could, repeat could happen for some Texas airports.