the next morning , when they awoke , the sun was high in heaven , and shone brightly through the leaves . then said the brother , " sister , i am thirsty ; if i only knew where to find a brook , that i might go and drink ! i almost think that i hear one rushing . " so the brother got down and led his sister by the hand , and they went to seek the brook . but their wicked stepmother was a witch , and had known quite well that the two children had run away , and had sneaked after them , as only witches can , and had laid a spell on all the brooks in the forest . so when they found a little stream flowing smoothly over its pebbles , the brother was going to drink of it ; but the sister heard how it said in its rushing , " he a tiger will be who drinks of me , who drinks of me a tiger will be ! " then the sister cried , " pray , dear brother , do not drink , or you will become a wild beast , and will tear me in pieces . " so the brother refrained from drinking , though his thirst was great , and he said he would wait till he came to the next brook . when they came to a second brook the sister heard it say , " he a wolf will be who drinks of me , who drinks of me a wolf will be ! " then the sister cried , " pray , dear brother , do not drink , or you will be turned into a wolf , and will eat me up ! " so the brother refrained from drinking , and said , " i will wait until we come to the next brook , and then i must drink , whatever you say ; my thirst is so great . " and when they came to the third brook the sister heard how in its rushing it said , " who drinks of me a fawn will be , he a fawn will be who drinks of me ! " then the sister said , " o my brother , i pray drink not , or you will be turned into a fawn , and run away far from me . " but he had already kneeled by the side of the brook and stooped and drunk of the water , and as the first drops passed his lips he became a fawn . 
because they would turn him into an animal .
why didn't the sister want the brother to drink from any of the brooks ?
why didn't the sister want the brother to drink from any of the brooks ?