Siblings
If we are accompanied by a sibling during a dream or in one of its scenes, it often alludes—particularly if the sibling is of the same sex as the dreamer—to aspects of the dreamer's shadow. From an early age, we project our shadow qualities onto our siblings. (Note that a shadow quality does not necessarily need to be negative, although it often tends to be.) As children, we are in some sense assigned roles in relation to our siblings—the eldest might be the orderly one, while the youngest is seen as careless. Yet neither is wholly one or the other, and "the other" can continue to appear symbolically as a sibling in dreams.
Sibling rivalry is, of course, universal and an archetypal motif. Family dynamics often foster rivalry, where one might feel that a sibling receives "more" from the parents than oneself; once again, "antagonism" emerges as a theme, lending itself perfectly to shadow projections and so forth.
Siblings typically share the same upbringing, which can make them fundamentally similar to oneself, both due to genetics and environment. In this way, passive siblings in dreams may evoke one’s background and formative environment, the shared experiences with a brother or sister that continue to shape one’s life.