@hawkeye10,
I am tending to agree with Hawkeye on this. Actually doing it would take some thinking beforehand on how to have it not go on and on endlessly, but be efficient in getting started, while not sending her up the wall re stuff she isn't ready to get rid of. That'll take patience. So - different kinds of piles - out out out, not ready to put out yet (move to side place), and want to keep for quite a while.
I know from my own doing of all this in my own house, that I transition from not being able to part with something (never mind the reasons) to being able to, not too long later. It's a process.
Plus the sister could probably use a big hug.
I pack my to go stuff up in boxes and call the Habitat for Humanity and their truck will come by to get them. There are likely plenty of places that would do that.
Putting items on consignment or selling on ebay (etc) is possible but time consuming. Having a garage sale - if she can have help with it. Craig's list, might be non trustworthy types show up, hard on a harried person.
Does your sister have friends?