My grandma died in 1990 - over twenty years ago. But I think about her daily. I am blessed that she gave me her Featherweight before she died. And I feel like she is always with me when I am using it.
When you use a piece from your mom's stash, I am sure that you will feel her presence and guidance. Actually, you will probably really enjoy that aspect of it. I know that is how I feel when I use my Featherweight. And since you have her fabric, you will know that she would love the project that you are working on.
You can never replace someone that you have lost and you never lose your memories of them. Think of her stash as little reminders of her as you get on with your life. I am sure that that is what she would have wanted. Se would have wanted to stash to be used and enjoyed, not just sitting in boxes.