So good news update - my sister made it through. There was some sort of exemption for people whose travel was already underway provided they stayed at an airport hotel, wore a mask, did not use public transport and did not eat in a restaurant. So she eventually arrived, starving hungry after meagre in-flight offering, a three hour wait to be border/COVID checked, have her phone checked, show receipts for hotel etc and no quarantine required. Thanks to all who have offered helpful suggestions, advice and information, and support/sympathy. Really appreciated at a stressful time.