STOP! DROP! and ROLL! You are shovelling xxxx against the tide and you are not likely to win. Take a look at threads started by "wife" and "teecher" in this section, and by "AJ" in the welcome to the new board section.
What I am going to say is my opinion only, and could be construed as unhelpful and inflammatory, but... 1) unless your mother in law is willing to change and both she and her family are willing to be involved with some counselling OR 2) There are grounds for, and you are willing to take-possibly without the support of your husband, legal action of some kind, I do not see that you can do anything to correct this situation.
I think the only thing you can do is to take direct, concerted action to see that you and your husband's own finances and living and storage spaces do not support and/or enable your mother in law's tendencies to excessive gathering. You could also attempt to get legally binding documents signed by the rest of the family excluding you and your husband from any legal requirement to fund future cleanup and living costs for your mother in law prior to turning control of the cash flows over to another family member.
The post after post after post I see on this site suggest to me that there is no pain free way the situation you describe can be resolved and it seems like a resolution that would give you some measure of recovery of mental and physical health is going to involve either withdrawal or confrontation.