Suggest she read, or you listen on audio together, The Life-changing Magic of Tidying Up by Marie Kondo. It’s pretty short and easy read. I personally really liked her method for handling the emotions of letting go of things and how to not feel guilty about it. Definitely helped me part with things that weren’t really sentimental but I had made them so.
Marie would also tell you that you can’t decide what your family members should or shouldn’t keep, each member has to make their own decisions on their stuff. This was frustrating for me, but it’s true that leading by example really works. Even though I felt I had less stuff than my DH, I focused going through my things and getting rid of my stuff. I would suggest my spouse go through his at the same time so that we could make one trip to Salvation Army, but tried (so hard, not always successfully) not to monitor what he was or wasn’t getting rid of. Sometimes he did, sometimes he didn’t, and it usually wasn’t as thorough of a clean out as I’d have liked when he did. But over a couple years as he noticed me getting rid of a lot of my stuff and not bringing more things in, he also started paring down his stuff more and he’s now much closer to where I thought he should have been at the start. It just took him longer to accept letting go of things, and realize the benefits of doing so. Now we’re pretty much on the same page. I learned a lot about patience, respecting his autonomy, and not being a well-intended bully in the meanwhile. :)