Estate agent here:
most contracts will say vacant possession by 2pm - sometimes it’s 12pm but 2 is the most common.
generally, you get 3 hours grace time , until legal issues can be started. So that’s 5pm best case scenario.
anything left in the house after this point, IS NOT transferred to the new owners and you will still have a legal obligation to dispose of it, even after they have moved in. If you don’t then they can and will pass on disposal costs to you.
tell your sister to get her ass back from soft play, and pack all day and all night. Even a large house can be packed up and be ready for removal in 24 hours. Put all boxes in a front living room, including furniture, or better still a garage or front lawn if it’s not raining.
pray that completion doesn’t happen until gone 2pm, which is pretty common with a long chain, and if your sister has the number of the new buyers, I would suggest she calls them and explains her situation. They won’t be happy but at least it shows she’s trying.
any costs incurred by the buyer, due to your sister’s incompetence can and will be passed onto her. This includes solicitors fees for additional contact, late removal charges, hotel stays - this can easily rack up.