Interesting thread here. The various versions of Lucy Wan, Fair Lucy, and Edward do seem to have many things in common which would hint at each version being a variation of the same antecedent. Where the incest theme is clearly stated in Lucy, there is no obvious reference to it in Edward, unless the allusion is allegorical. In the stanza

It is too red for your old coon dog
My son, now tell to me
It is the blood of my brother John
Who hoed the corn for me, me, me
Who hoed the corn for me.
What did you fall out about?
My son, now tell to me
Because he cut yon holly bush
Which might have been a tree, tree, tree
Which might have been a tree.

"Hoeing the corn" may be a sexual reference, and "cutting down yon holly bush" may be alluding to killing the sister. This Edward version would appear to be of Appalachian origin, while Lucy appears in Child. Does this indicate that Lucy was cleaned up for the tastes of the Scotch-Irish in America?