Who has the rights to deceased mother assets if she has only been married with stepfather for 1 year and has nothing in he’s name?

I need some answer to so many questions do to my mother death. The situation is getting ugly day by day with my stepfather trying to keep all of her assets to himself and he’s son which my mother did not adopt. My mother has all vehicles, house and furniture in her name and other relative that is deceased as well. What is our right for my brother and I as children? Can he sell the house if we don’t agree? Can he prohibit us to take our mother’s belongings? I know is 1/3 to each one for including my stepfather. There’s nothing in he’s name and he does not have legal permit to be in the United States. What can we do to safe my mother’s assets from my stepfather that’s trying to keep everything without us agreeing to anything?