Original Essay:Grimm's Brother and Sister struck me initially as being very similar to the Russian skazka collected by A.N. Afanas'ev, Sister Alionushka, Brother Ivanuska. The similar tales from different cultures points me to the universal aspect of this tale - the Cycle of Life.
Water, which I would associate with Life, is a motif which recurs at two major points. Brother's thirst at the beginning is emphasised and his impetuousness in drinking the 'wrong' water is the cause of his transformation into a passive Fawn - maybe a punishment for pre-empting movement in the lifecycle. Water is then the place where Sister (as Queen) is murdered and then preserved (to wait for the Male agent).
The second aspect that caught my attention was the woman's role being attributable to a stage in the Lifecycle, referring to the three aspects of Maiden, Mother, and Crone. At the start, Sister is the Maiden and her Step-mother (Witch) is Mother, and there is no explicit Crone (it could be interpreted as the dead mother). The Witch leaves Sister alone until as Queen, Sister is pushing the Witch forward from Mother to Crone. The Witch's Daughter is introduced as the new Maiden, but wishes to change her role without the male agent.
Men have the roles of agents - enabling women to move onto the next stage in their lifecycles. They are not active in their own right, but only as agents for the change of the women in the tale. However women who try to move without them are punished.
To me all the Grimm tales have an underlying moral, and this one seems to me to be saying 'Do not usurp a position in the Lifecycle' - life carries on and we must move with it, but also "Accept your role and don't try to move yourself"