I could believe the brother recognized Uncle Ted's writing. Hell I've recognized the writing of people I know IRL. I was reading an article once and thought to myself "this sounds a lot like X", and when I looked to see who wrote it sure enough he was the author.

That's one of the reasons I think it's probably fairly easy to do. People use particular phrases that others don't or use certain words to describe a thing more often than available synonyms. A simple system would probably just involve taking a large sampling of an individual's writing, performing a simple frequency analysis of words/phrases, and identifying outliers compared against a general corpus of writing.