A child who is truly attached in a healthy way to a sibling probably does not have huge attachment problems.
We can agree to disagree on this one, I based my opinion on therapy we've been through and the RAD children we've had. RAD is related to a child's CAREGIVER and the history of CARE. True, attachment can be transferred and all that, but whether or not a child is attached to a sibling or a pet or a favorite neighbor does not have a lot to do with how he can attach to the people he should trust to provide his basic needs.
For example, my RAD son was VERY attached to a foster child in our home for over two years. I worried the child's move would trigger my son. His therapist assured us it would NOT, it is a child, not an adult or caregiver leaving. I was not convinced. In the end, he was appropriately sad, it did not trigger any RAD behaviors.