Dreaming of siblings can be pretty ordinary stuff. If you have brothers and/or sisters in waking life, it would be expected that in dreams of your family, they would be present. The important work of interpretation comes when either your sibling is missing, or if you suddenly acquire siblings in dreamland that do not ordinarily exist in the waking world.
If your real-life sibling is noticeably absent from a dream event, it may reflect your perceptions of your family at large. While either you or your sibling may be at odds with the family, a dream of an incomplete family is a picture of fracture in one or more of the possible relationships.
Sometimes, if you are experiencing particularly strong identification with a co-worker, they will show up in dreams as a newfound sibling. If the dream solution to the new sibling is a comfortable arrangement, this may herald a positive partnership. However, this event can be a little troubling.
In dreams of this nature, you may feel as though the new sibling is burrowing too far into your life and relationships.