There are plenty of reasons for concern. Many also worry that one or both parents may get cancer. The diagnosis of cancer changes children's views that the world is a safe place. They feel vulnerable, and they are afraid. Depending on their age, siblings worry that their brother or sister may get sicker or may die.
Some siblings develop symptoms of illness in an attempt to regain attention from the parents.
Many fears can be quieted by accurate and age-appropriate explanations from the parents or medical staff. She developed many fears and had frequent nightmares. We did lots of medical play, which seemed to help her. I let her direct the action, using puppets or dolls, and I discovered that she thought there was lots of violence during her sister's treatments. She continues to ask questions, and we are still explaining things to her, four years later.