> is it not possible, even probable, that this sequence of family members coming to someone's aid originated separately in different cultures?

I would certainbly go along with that if we only had the English and Romanian texts to go on. The possibility of direct borrowing between these two ballad traditions would be pretty remote. But given the various analogous ballads in different languages that Child lists, to which Romanian, and also (I understand from reading notes on the Romanian ballad), Hungarian can be added, doesn't it make sense to see these as local variations on one pan-European ballad motif?