I confess to not knowing, but I'd have thought so called "Acts of God" no one were liable for. Different service providers may have commitments built into their licences and stuff, like finding accommodation, but that aside sometimes life sucks.
Also unless you have especially searched it out, "Acts of God" are often used by insurance companies to renege on their commitments and are mysteriously not covered when you try to claim. But worth trying anyway, you never know.
You might hope for a class action on the airways and/or government if this proves to be an over cautious clamp down, but personally, I wouldn't hold my breath. Maybe this is the "rainy day" your sister will have put funds aside for ?
But in the past I've been more pessimistic about situations than it turned out I needed to be, so who knows ?