Wiki on "Seven Ravens" says that the Grimms changed it to seven brothers/ravens. "In the original [German] oral version" (probably the one that the Grimms collected) "there were three ravens". So the Grimms made the heroine's silence period longer; the condition is always that the heroine stay silent for as many years as she has brothers. Probably this was done to create suspense? By lengthening the silence period, the reader can wait to see how long she will comply with it. Something like that. There is also a Greek version of the same story.