Unpleasant dream. Could anyone help with its implication?

I wouldn't put much stock in dreams. They are usually just a replay of what is preoccupying the dreamer. I won't go into the separate theories, but research has shown that dreaming has no provable precognition or telepathic powers. If the daughter has had such a vivid dream about this subject, then it probably means that she is preoccupied with the situation, and that she feels guilty about how she has been treating the elder brother, and the dream reflects the feelings and events of the day.