Bear in mind that in dreams, siblings can also represent the shadow. This is because it is easier to project the negative or hidden side of our personality onto another member of the family than it is onto a nonrelative, and siblings tend to be the closest to you in age and experience. Your dreams may be an attempt by your unconscious to resolve this projection before it creates problems in waking life. When dreams of the opposite sex sibling occur, this may also be an expression of your anima or animus archetype. It could represent the tension—or union—of opposites in some way; your dreaming mind may be trying to create harmony and balance in your life by pointing to some aspect of yourself. For example, if your dream sister was incredibly rude, the dream may be highlighting your own feelings of guilt about being rude to some one.
Alternatively, such a dream may be urging you to be more assertive in waking life. The same would apply if you dream of siblings or step-siblings but don’t have any brother or sisters in waking life, or if your sibling behaves completely out of character in the dream.
If your dream casts someone you know as your brother or sister, this may be because you have brotherly or sisterly feelings for them, with all the associations of rivalry and platonic love that such a relationship implies. [1]