We have a fantastic guide called Trans Summer School: Let's Bust Out of This Closet!
which I think will provide some answers to your questions.
In the article are: a guide you can give to your parents, suggestions on how to talk to family members and questions you might hear from them, suggestions for helping you plan going forward, and importantly: help for how to set boundaries. You have the right to set boundaries for the type of language that people use around you, and for how you want to be addressed.
I'd check out the guide to talking to your parents, and see if you can leverage their support here. "Mom, Dad, I know this is a change for you too. I appreciate you trying to be accepting of me. [It's unclear to me how accepting they are being, because they've dismissed your concerns, but if they're using your name/pronouns you could mention that you appreciate that so they see that this is important to you
]. I'm struggling because TwinBrother seems to be purposely using the wrong name and pronouns for me. Every time he does this, I feel really uncomfortable. He won't listen to me. Being called the name and pronouns that feel right to me is super important for my mental health. Can you please help me out? I need your support here."
For your brother, you could say something along the same lines: "Hey, I know this is a change for you too, but it's definitely a much bigger change for me. When you deadname me, I feel deeply uncomfortable. Being called the name and pronouns that feel right to me is super important for my mental health. I know mistakes happen sometimes, but I'd feel really loved and supported by you if you would make a sincere effort to try."
What do you think? Could saying either of those things help?