Two notes on "Two Brothers".

Bert Lloyd wrote, in the sleevenotes to Bellamy's The fox jumps over the parson's gate:

"The song has its relatives not only in Britain but on the continent too, and tracing its sundry versions we find tha tit concerns not merely a violent bit of schoolboy horseplay but a murderous quarrel over a patch of land, and beyond that, in the oldest versions of all, we find that the root of the dispute is in incestuous jealousy, with both brothers enamoured of their sister."

Emphasis added. None of Child's texts feature incestuous lust. This may be an element in some version somewhere; I don't know about "oldest".

Then, following leads on another thread, I found this at Jack's "Embro" site:

"In July 1588, William, one of the sons of Somerville of Drum, accidentally killed his brother John with a pistol; his father at first thought it was murder and William had to flee. The family never recovered from the tragedy and lost all its wealth and power over the next few generations. Folk tradition sometimes declares it was murder prompted by rivalry over a girl, and almost always has the killing done with a knife. The more precise and detailed a version of this ballad is, the less it has in common with the historical events."

I'm inclined to believe this; I think lurid detail is more likely to attach itself to a story over the years than to be lost, as Lloyd's suggestion would require.