Did sister ask why were they asking for keys if they were no longer dealing with the apt?
Did she contact landlord?
Is mngt co a licensed auctioneer or a small private two-man office or what ?
Sounds iffy. And if there is any dodginess going on then then you must make as much fuss as loudly as possible -immediately and continually. PRTB take ages but ,yes, should be contacted.
Returning keys or not probably won't make much difference, so don't depend on them having much hostage value. Landlord probably has a set and will change locks.