Your sister sounds like my vendors, who went on holiday to Spain for a week, returning the day before completion having only packed up a few boxes. Literally most of their possessions were still in situ when we arrived with our van, and they only had a mate with a small transit helping to clear a 3 bedroom house.
We sat outside with the removals van for an hour, then went inside and asked what the fuck they were playing at. It was chaos - two dogs running around, boxes everywhere, they were sitting there drinking tea!
In the end we just told our removals guys to take our stuff into the house and they kindly helped out by removing the vendors' possessions to the front garden. They took bloody hours - we were fuming, it was 10 days before Christmas and freezing cold and dark by the time they finally cleared the garden and left.
Cheeky bastards hadn't cleaned anything, the place was thick with dog hair, fag butts all over the garden, and half of their kitchen stuff still in the cupboards, things in the loft and garage.
They turned up a week later asking for some specific kitchen items, not even everything they had left behind, and said they had forgotten to clear the loft, could they 'just pop in'? I told them we'd assumed they hadn't wanted anything left behind and it had all gone to the tip (it had). They also hid a set of broken ladders behind a neighbour's garden fence which wasn't discovered for weeks!
We received post for them for 8 years because they hadn't redirected anything.
Absolute nightmare - I'm still angry about it 19 years and two houses later!