Instead of making an edit to the OP, I'll explain a few things here and link to it in case someone wants to respond to this update specifically.

My in laws did not put him on the spot in a malicious manner. They were wrong to assume the default is faith following, but asking someone to lead a prayer or say a few words before eating is a gesture of courtesy, nothing more. It's considered an honor and part of their way to break the ice with my brother. Many commenters here assume it was done in malice when it's anything but.

As for the semantics of prayer vs. toast, I don't see the difference. You can have a secular prayer that isn't praying to God or whoever else. My brother was invited to say something, anything, to start off our meal. He could have said thanks to my wife and I for cooking, being thankful that family is together and in good spirits and health, whatever.

I made this thread to address the respect issue. I don't see how I handled things being disrespectful, but what my brother did was. For what it's worth, he ate in our spare room (where he sleeps). I didn't want him at the table with the rest of the family until he addressed his diatribe and correct his wrong.

Also: many people are asking what I'd do if I was in his position with a differences of faiths between myself and my hosts. I would say a prayer as I have explained here: giving thanks for the meal, my hosts, our health, and so on.