It's possible that half sister (and possibly also your GF) equate stuff with people. Not that stuff is people, but that it's a direct physical manifestation of the deceased person (or living person). My first BF had an assortment of things that had been his mother's and grandmother's. His apartment was jammed with it (but very clean and neatly organized.) All that stuff gave him a sense of still being close to his mother and grandmother, both when they just lived in another state and later after they had both passed away. It was like he literally felt his grandmother's presence every time he sat in her ugly living room chair that she had sat in for many many years. Had he not passed away as well I'm sure he would've kept that stuff for decades.