Sandrine,
If I was you, I would consider taking the personal things that are very important to you and to your relationship with your bear and storing them in a safe place. Those things that he had before you and that really were not a part of you and your bear...perhaps you should just give them to his family. If there are items that they want and you cannot part with them, tell his family you are not ready to let go.
My situation is somewhat different from yours. I was married to my second husband, but we had less than 4 years together. There are many things that my husband's brother wanted. I told him to make me a list, and when I was ready to go through my husband's things, I would make sure he got the items that he wanted if I chose not to keep them. It took me 2 1/2 years to be ready, but his brother got almost everything he asked for and much more. My BIL seems satisfied with how this worked out.
Your bear's family grieves their own loss and in their own minds, they were family to him. My first husband's mother told me several times that losing your child is harder than losing your spouse. In her mind, this was the absolute truth. It hurt me each time she said this, until finally one day I stopped her and told her that they were two different things and we were both grieving and it didn't have to be a competition. That made her think somewhat differently. As much as you may know that your bear was not taken care of so well by his family and that he had the best years of his life with you, they see things from their own eyes. They have lost their son and they grieve. They want to hold on to their memories of him, just as you do. They may not be able to understand your bond with him and how you were the most important family to him. You know this. You were the person he chose to live his life with.
I wish that I could tell you how to address this situation easily, but it is not easy. Sometimes we have to find our voices and try to gently tell people that their words and actions hurt us. You may need to tell his mother and stepfather that you continue to grieve heavily (just as they do) and you are not yet able to consider letting go of his property. Assure them that you will not throw anything away, and that, in time, you will consider their requests. I imagine that in the future, you will be more ready to give them some of what they want.
Hugs,
Maureen