What’s strange about this case is that there are lot of carcinogens and evil pesticides out there that are not heavily regulated. For example processed meat has a very strong link with cancer and the nitrites added to processed meat are likely the culprit. No one seems to care and no one is suing the meat companies. There are “export only” pesticides that are truly scary that contaminate the us food supply and cause birth defects and cancer for entire villages. Roundup isn’t mechanistically tied to cancer and there isn’t a scientific consensus surrounding if/how carcinogenic it is. I wouldn’t be surprised if your brother’s cancer was environmentally related but I would bet that a boring chemical like gasoline/exhaust would be the cause. Gasoline and exhaust products are known to be carcinogenic.