My sister always think shes right?

Yesterday me and my sister got into an argument because of this she told me I couldn't go into the basement which I left my backpack down there. I tried to tell her that it was down there but she didn't even let me finish and she handed me my backpack from last year and told me that I needed to do homework. Today I asked her if o could get my backpack from the basement. She said oh so you've been in the basement without my ******* permission! I've tried to tell er everything she wont consider other possibilities or listen. What should I do?