Thanks for sharing this. Honestly I doubt there may be a lot you can do to get help for your brother & to get your parents to stop yelling. You wrote that your brother would never go to a counselor. So that precludes one of the primary ways in which problems such as you describe are addressed. And you wrote that even if a professional spoke to your parents they'd be right back to yelling within a few months. So my inclination would be to say the best thing you can do may be to take care of yourself & continue to be there to support your brother as best you can.

Here are links to 3 articles, from PC's archives, 1 that discusses contamination OCD in particular & 2 that discuss treatments for OCD in general:

OCD: Symptoms of Contamination Fears

Obsessive-Compulsive Disorder (OCD) Treatment | Psych Central

ERP Therapy: A Good Choice for Treating OCD

I don't know if this would be a good idea but one thought that comes to mind, with regard to your brother, would be for you to get one or more OCD workbooks that the two of you could work on together. I believe there are some available on Amazon. Perhaps other PC members might have some recommendations for particular workbooks they feel are beneficial. Perhaps there might even be some way of involving one or both of your parents in this & doing so might help to open their eyes so to speak as well. Anyway... just a thought.