Invite other people that he respects into your house and have him listen to their reactions.
If he doesn't respect my opinion on the hoard, he's definitely not going to respect anyone else's (maybe a hoarding expert, but he would never agree to have them come over anyway because he sees telling other people as "airing the dirty laundry"...he would be PISSED if he knew I was posting about it online).
When pressed he does actually admit there is a problem with her behavior and with the state of the house. That's an improvement over a couple of years ago when his parents could seemingly do no wrong. He just feels obligated to be nice to her and enable her because of everything she did for him and because she almost died, and because he feels responsible for his father's death because he thinks there was some way he could have gotten him to go to the doctor.
Also, the house is actually in my husband's name now (another reason why she can't move back there, if anything happened to her he would be responsible). It was signed over after she got sick. He is cleaning it up but he doesn't have a lot of free time, so it's happening very slowly. But it isn't really a super urgent concern of cleaning it up now, and since nobody is living there, calling in the fire marshal and health department seems a bit extreme (also they would not care). Originally he wanted to renovate it and have us move in there, but he has decided to sell it for lot value after looking at the costs and time involved. However, this creates a problem because when I mention that in the future when we have kids, we may want to buy a house, he says "I already have a house. I don't want to spend money on another one." Like ok but we could get a nice house with working utilities in the nice town where we live, not that piece of crap garbage hut in a gentrifying low-income neighborhood.
We have no choice but to store certain things of hers from the house. Family photos, important papers, and stuff like that. There just isn't any other place to put it and they would never trust putting it in a storage unit. However, I've been preventing stuff like furniture and other items coming into the house. Whenever I mention that I need to buy something for our apartment, she always says "Oh, we have so many of those in the house!" Like, yeah, I want my own stuff, not your decrepit, pooped on by animals stuff, and I don't want to feel obligated to you for anything because I accepted your secondhand crap.