Hi, Tiger.
This is perhaps the most I have ever empathized with a post. You sound like me. My older brother is my abuser, and he lived with my parents up until about a year and a half ago. I live 800 miles away from them... by my own choice for sure.
My brother is gay. When I disclosed my abuse at fourteen years old, he was 20, and no, my parents did not want to believe me. I snapped at sixteen, dissociated, wrote down several episodes of my abuse in horrific detail, and stuck it under their door. Then they were listening. I told them that I was moving out when I was seventeen (about 4-5 months from when I issued this ultimatum) or he was moving out. He was out in about a month and a half.
When I left for college, he moved right back in. I don't know how many times I had to tell them, but I did, that if he was there, I would not be. I thought maybe I could tolerate it at first, spent my first summer in college home, and it just destroyed me. It damaged me so badly that I am still living with the recklessness and self-harm I exhibited that summer. So I laid down the law. I'll come, but he has to be gone.
I know I'm simplifying things, and I don't mean to, but I think it's important that if you will not disclose your abuse, you at least tell them that you aren't coming if he is going to be present. It's not that much to ask. If you're 35 and he's your oldest brother, I imagine he is an adult perfectly capable of fending for himself now and again. They don't need to know more than that you will not come if he'll be there. A therapist could guide you through your decision to disclose or not disclose your abuse; that's for you to work out. But I strongly recommend that something's gotta give in this situation. I am concerned about what these experiences will do to you, now or down the road.
Please know that you're in my thoughts. It's difficult to respond to you because this situation reminds me so much of my own, and I feel all the emotions that drags up. I empathize. I really do. I feel for you.
Big hugs if you accept them. (((Tiger))) Please put yourself first and remember that your truth is the only one that matters in the end.
Edit for a postscript: I totally understand you when you say that he is still powerful. I'd like to suggest how to break down the power he has over you, and I will if I ever figure it out myself.