Have you trained them to be okay there or are they just shoving them in?
My cats are crate-trained, so yes, I could. First I feed them in there, then, I practice shutting the door briefly and opening it. Then a little longer.
I have not taken it further, but your sister needs to practice ahead of time, not the day of. And decided on whether or not to get one or two large dog crates with fluffy pillows inside and group compatible cats or individual crates (with individual fluffy pillows).
The may get very stressed with a party going on, in or our of crates. If she does crate them, she needs to make sure they remain undisturbed by guests.