Joe - Mostly Child seemed to be saying "these are song 1 and these are song 2" with some attempt to group related songs that he saw at about the same time into adjacent numbers, so it not a hierarchical system (like Laws with definate groups). Child roughly grouped magical songs, border sallads, incest songs, robin hood, etc. but then added others he found later into higher numbers. It is not always clear to me why some related songs are lumped into 1 number and others are split into 2 or more.
Sandy - yes, to some extent the Two Brothers could be a prequel to Edward, giving some reason for the fratricide. And in some versions of the Two Brothers, they both love Pretty Susie, giving it some tie to Rolling of the Stones, but hardly a strong one.
Sophecleese - In the Unquiet Grave the living lover mourns at the grave for a year and a day (not magical) disturbing the dead lover and Pretty Susie charms the dead lover from his grave (magical) right after he dies. Again, some relation, but no real strong.