We’ve only ever planned to list the story names, since there’s the chance that one of the AUs gets ignored in favor of the others, but I can at least give you a hint.
Back when the story excerpts started, I put in a hint as to the name of the story that each excerpt is from, and Brothers Lost was the first AU that was featured. You can figure out what the story name is for that story, and any story written afterwards, by reading the tags. Check the link below to read them from the beginning:
Sadly, I didn’t think of story excerpts until after we wrote Brothers Found and Brothers Adopted.
I’ve got my fingers crossed for Brothers Lost, personally, but even we have no idea which story is going to win!