The fact that this reference is so specific to the point that no one has caught it to this day and even people who know the ballad haven't caught it either, makes me think that while writing this part of the movie the writers were like: "Yo, we need two people to help Jack in this scene, they have to be fairytale characters, who do we choose?" "Hansel and Gretel?" "No, too much obvious." "Jack & Jill?" "No, we already used them in the first movie, does anyone here know any other fairy tale stories with simblings?" "Well, there's this ballad with two sisters who kill each other." "Does anyone else here know any fairy tales with simblings? Friends or anything like that? Nobody?! Fine them, we will go with the killing sisters" Like, I'm 99% sure they only put those two in this scene due to lack of fairy tale characters options.