Ok, I stand corrected. There are several versions that include both the unintentional killing and the charming and I did not see the stepmother anywhere. I looked at the 7 versions of Child 49 in child, 11 in Davis (Traditional Ballads of Virginia), and 1 in Linscott (New England).

Child A: brother kill, say I'm dead, no charm
Child B: brother kill, Lady Margaret harps him out of grave
Child C: brother kill, lover weeps him out of grave
Child D-G: brothr kill, tell them..., how comes the blood...
Davis A, B, F, H, K: brothr kill, Susie charm, kiss
Davis C, G, E: brother kill, tell her I'm dead, no charm (name Little Sweetie in G, E)
Davis D: brother kill, half verse on charm not tied in
Davis I: charm, kiss, little else left
Davis J: short, no lover
Linscott: no brothers, just Susie & lover & charm

so, of 19 versions so far
charming from grave in 2 versions
otherwise moved from grave in 2 verisons
brother kill brother in 17 versions
kiss or clay in 8 versions
how comes the blood in 4 versions
stepmother incited the murder in none
I'll look at some more and see if I can find where the stepmother came from.