I am very lucky in that while my mom has, over the years, tried to foist stuff off on me, she does it in such a way that if I say no, she's ok with that. I get right of first refusal, and sometimes its something I may want or need, in which case I take it, and others, I can simply say no, she doens't get upset or offended, and will then donate to her charity/cause/event of choice. I used to laugh at her as the big joke was what will I bring home next?, but only if I wanted to.
When she lived in my state, there was a local organization that had a huge sale every year, and she used to donate up a storm. Now that she's moved, her church has a similar, annual sale, so stuff goes there.
It started for her when my great aunt passed away, and she inherited everything. Some things my grandmother took, but when she downsized, and then went into a nursing home, mom ended up with it all. As did I. Mom calls my apt "early, middle, and late attic" as aside from my bed, all my other furniture came from her, grandma, etc.
She also downsized several years ago after my dad passed away, and I think even she was amazed at how much stuff she had. she was good though, about selling, donating etc. most of it. The big joke was my inheritance was several pieces of furniture, but even now, I'm on the fence about them, as I simply don't have space and don't think i want to put them in storage.