This is all speculation on my part, but I suspect someone identified the younger brother from the photo and then the FBI went back and "put two and two together" to identify the older brother from their own records.
re: “This is all speculation on my part, but I suspect someone identified the younger brother from the photo and then the FBI went back and “put two and two together” to identify the older brother from their own records.”
Yes, that sounds very plausible. Sorry, all the “conspiracy” stuff gets to you sometimes. Thank you for some clear thinking.