My MIL came to live with us last May. She had suffered a stroke and while physically able, she has short-term memory loss and mild dementia that prevent her from living alone. We have a mother-in-law apartment add-on with a kitchenette, bathroom, a tiny bedroom and a tiny living room. There is very little room for furniture and it contains a twin captain's bed, a recliner, a table, two chairs, a large book/storage cabinet, and an end table. She is unable and unwilling to make any decisions about a full household of stuff sitting in boxes in our garage. We had to move everything over, including the junk, and much of her furniture is stored at her daughter's house. Since moving in, she has added a cabinet, a bookcase, a large packing trunk, and various odds and ends into the apartment, which is now over-stuffed. I fear it will eventually become a storage shed. Part of her unwillingness to part with anything is due to a belief on her part that she will eventually "get better" and move to her own apartment. Unfortunately, that will never happen. In the meantime, we have lost the use of our garage, which is full of boxes. What I would like to see happen is to get rid of much of the day-to-day items (kitchen stuff she will never use, duplicates, and old bills and the like.) I would want to keep for posterity her memories (photos, cards, letters, antiques). She frets constantly about her stuff, but can't make decisions about any of it and always ends up crying. Do I just learn to live with it? Sneak it to the Salvation Army or the trash? How to other people deal with this. We have spent a few years down-sizing ourselves, so it is difficult to face a mountain of my MIL's stuff.
One thing I'd like to do is to be able to go through those boxes one at a time to try to figure out what is worth keeping for any reason. I know it would be a huge job, but maybe you and your MIL could try going through them, a box a day or something, so she could remember what she has and make a leisurely decision about what to keep. Part of the reason I'm so attached to my boxes is because I just don't remember what's in them!
I find that my mother is just as attached to her own stuff even though she hasn't had to move house. She's deathly afraid someone will come in and get rid of something without her knowledge. Sometimes I sneak something I'm pretty sure she'll never ask for again into the trash, but she seems to have an uncanny sense about that, so I have to make sure it's something I can either replace or explain away its disappearance if need be. We both know they're just things—but when you've already lost so much you sometimes need to hold onto something, no matter how illogical.
I hope it works out for all of you—and you're certainly to be commended for trying to make things easier on her.
I make divesting decisions on a lot of stuff, and I don't lie about it, but I don't offer the information, either. This is for items he won't notice being gone, like non-heirloom china/ dented flatware, papers he doesn't need, that sort of thing. I don't toss any of his HUGE toy collection, unless it is a health hazard, but it too, is going to have to be downsized, and I am trying to get him involved on the decisions for that. The VA is looking into getting him therapy for his hoarding, and I am hoping that will help us not only gain more living space, but become better partners in our walk.
But for your situation, I think your husband is going to have to help you decide what to divest. After all, this is his mother! One thing, does your MIL have a focus? Activities, ways she can spend her day that don't involve adding more stuff to her stash? You said she is physically able. Having a focus for her energies would help greatly. That's what my hubby needs, a reason to stay off of eBay! :)