@journierman I'm sorry for your loss, nothing will ever replace your mother.
As you probably guessed, your uncle thinks that now your mother has passed, he can just take over.
While I know you are grieving, you will need to give uncle some tough love and you will likely need to act fast.
You are the majority shareholder, you were next in line. He needs to like it or lump it. He might give you some 'I am your elder/I am a man bs', but you need to remain firm. If he doesn't like it, he can leave. I mean as the majority shareholder, you can decide who is in charge, no?
I would demand he return everything to your mothers office - who gave him permission to do that? Nobody.
You could also ask your workers to not respond to him, ignore him, hes not the boss. If you work with other companies, let them know you are taking over and that you want to continue your working relationship.
While he is family, I'd consider firing him or putting in a boxed role.