It sounds like you are likely to be criticized by various members of your husband's family if/when you get rid of the stuff. I'd suggest that you might preempt some of that criticism if you make a point of giving a deadline for the removal of the stuff - and then telling lots of family members about the deadline. This will make it harder for those who left the stuff behind to whine that they would have retrieved the stuff if only you had warned them.
I'm remembering a story about one of my relatives. (I'll call her "Gracie".) Gracie's mother passed away many years ago when Gracie was a young adult. Gracie's father was devastated at the time and just left Gracie's mother's belongings where they were in their home. (He offered Gracie's mothers things to Gracie, but Gracie never got around to moving any of them.) Well, after several years as a widower, Gracie's father married a very nice lady (let's call her "Sarah"). He and Sarah decided to live in his house after their wedding, but there was a problem: all of Gracie's mother's things were still there.
For years now Gracie has told the relatives that her father and Sarah just tossed all of Gracie's mother's things without ever giving Gracie a chance to have anything. I found that hard to believe, because it didn't match up with the kind way Gracie's father and Sarah have always behaved toward me and my own family, but of course I just kept my mouth shut and listened without comment when Gracie would whine about it.
Well, I stayed behind at the end of a recent holiday gathering to help, and I ended up having a long private conversation with Sarah. Sarah asked me point bank, "Does Gracie say bad things about me to you?" I avoided answering the question directly and replied by quoting some of the nice things Gracie has said about Sarah recently. Well, Sarah went on to tell me that other relatives had been telling her about Gracie whining to them about Sarah tossing out Gracie's mother's things. Sarah said to me, "You know, Snappy, Gracie's dad offered all of those things to Gracie when her mother died. Gracie would never come get them. Then, when her dad and I planned to get married, her dad sat Gracie down and told Gracie he would not have room to keep her mother's things any longer. He gave Gracie a deadline four months in advance and told Gracie that any of her mother's things that were not removed by the end of the four months would be given to other relatives or to Goodwill.
Well, Gracie never came to get any of her mother's things! Sara told me that her husband even waited a few extra weeks after the deadline, but still Gracie would not pick up the stuff. Finally, Gracie's dad donated it all to Goodwill. And then Gracie started complaining to the other relatives that her mothers belongings had been tossed out before Gracie had a chance to get them.
Now I've heard two stories, Gracie's story and Sarah's story. Frankly, because of what i have observed over the years of the way the two women behave, I believe Sarah's version over Gracie's. (Not that it matters what I believe!) In any case, I'm suggesting to the OP that, if you're able to let all the relatives know about any deadlines you set, it may make it more difficult for the ones who get upset with you to distort the truth around the others.