John G, I can relate to what you're going through. I am an only child so the "going through" task fell completely to me. But at least there was no one to argue with.
I hope you and your sister know of a few good thrift shops. My mother had so may clothes --she'd never get rid of anything -- that I split her clothes up among several shops, as I figured there would be too many for any one shop to handle. She used a deep closet in the den, as well as several other closets. When I went through the den closet it was like the magician who keeps pullling scarves out of the hat -- neverending. Another box and another and another. Some probably hadn't seen the light of day in twenty years.