Hi. I'm so despondent about this - so many years of pain.
My brother (now in his 40s) has mental and (poorly-defined) physical health problems, and substance addictions. Also definitely some undiagnosed neurodiversity, in the context of very high intellect. He also emotionally abuses and controls my parents horrifically. They are terrified of him, and their whole lives revolve around his needs and fear of what he may do to himself. It's been going on for at least 2 decades.
As they get older, it's increasingly breaking my heart. They can't just leave him, as one would advise for a romantic relationship, because they are also his carers and desperate to keep him safe and to help him.
Whenever I visit them, he has to be involved (my parents get very upset if I don't contact him, because they say how lonely and easily hurt he is). Every plan has to take him into account (often only subtly, as they hide this). He monitors and controls them, and demands their involvement in everything. He diminishes them, and intimidates them. I love my parents so much - they are amazing, caring, intelligent and thoughtful people. They also do know that they're being abused. I love my brother, too, and know he suffers horribly. But I also hate him so much for what he does. It's getting worse. It makes me not want to see my parents, because I can't stand his involvement and endless control/monitoring. But also, if I try to visit and not involve him, I don't really get to properly be with my parents anyway, because they are (covertly) miserable about how upset my brother would be (and I know he will subsequently make them suffer).
What can I do?