I’m so sorry to hear your sibling are essentially bullying you. To be honest, I know the horror and defeatism that comes with siblings being rude about your stutter. My brother would literally be up in my face and mimic my stutter while I was stuttering. It scarred me and I’m barely taking steps in my mid- thirties to overcome it. My advice to you is to be patient with yourself as you will get to a point where it’s manageable. That may not be what you want to hear but it’s what’s helped me. Practice mediation and deep breathing techniques to ease your anxious mind. Lastly, get a good nights rest and practice words that you find yourself stuttering over. Godspeed.