I'm really sorry that you are in this really painful position. It sounds as if your parents have to reach a point where they have had enough as well. Perhaps if they see the serenity you find by seeking help for yourself in this situation and detaching from the insanity of addiction, it may help them to find ways to help themselves too.
I found going to Naranon meetings really helped me when my daughter was actively using. I was fortunate that the group I found had many parents as well as siblings, and we could really relate to each other. That face to face support, plus coming here, truly was life saving. As other family members saw the changes in me, they started to work more on themselves too since they could see how much it helped.
One suggestion I have is to consider telling your parents that you would be happy to see them outside their home but for your own well being, you can't be around your actively using brother. And maybe that time together also includes no discussion of him or his problems? Since you can't change them, you can protect yourself and they may even find that this gives them a needed temporary escape from the insanity.
It's hard to not feel guilty, but you are not abandoning anyone. It is okay to disengage, and hopefully you will find peace in your decision soon.