Pip Radish raises an interesting point about the relationship between these ballads. It is worth noting that all of them are very scarce ballads. You could say this was possibly because of their subject matter. OR there could be other reasons. One could certainly have inspired all of them and they are riddled with commonplaces and have whole sections which could have been taken from other ballads as often happens. I'm not convinced about Child's attempts to relate them to Scandinavian ballads, but to be fair he is only pointing out similarities. More than one of them is first found in Herd's manuscripts and the others could easily have been fabricated from these. If we bring Motherwell, Buchan and Scott into the equasion, these are not always reliable sources. (IMHO)

ChillToad,
The tree couplet is from 'Edward' (Spits in disgust) and has only been interpolated into hybrid versions of the Two brothers. It doesn't occur in the incest ballads and there is no need at all to imply any symbolism of any sort. It is simply a way of saying he killed his brother over something trivial (IMHO).