In their minds, they did email their brother, and the weird guy who replied is wrong. It might be possible, with enough investment of time, to help them understand how/why it went wrong, but as nicolas_t experienced, it is sometimes easier just to block them and move on.
I was lucky, I was a Hotmail early adopter and I regularly receive emails for two namesakes. I was able to figure out their correct email addresses over the years and they are both tech savvy enough to understand the issue. I forward on messages when they come through, and I actually met up with one of them at a house party he was invited to.
When I forwarded the invitation, he suggested coming along as well and I figured why the hell not. It was a fun ice breaker in a room full of strangers.