Apparently, there exists more than one AU in which
Jennifer Taylor marries Jack Kinney, thus making Brian and Justin stepbrothers. My suspension of disbelief fails to kick in at the idea of Jennifer being tasteless enough to marry Jack.
However, the idea of Joan Kinney marrying Craig Taylor doesn't seem as far fetched, especially if, as AU's often do, one minimizes the age differences between all the relevant parties. So, does such a fic exist?
ETA: Despite the high comment count, no such fics have been found--yet.