Thanks, rickye (post 39). I think your interpretation of the puzzle title is correct: the relationship of the members of each thematic pair could appear as "one across" in a simple family tree.
(Another pedant alert. In 31ac "Brigadoon" is a [ b ]mythical[ /b ] village in Scotland. A better choice of musical would have been "Hamilton" — it's a town very close to Muttergut!)