It is logical to them to feel that since their sister has cancer, they caused it. This notion should be dispelled right after diagnosis. Children really need to be told, many times, that cancer just happens, and no one in the family caused it. They need to understand that just because you think something or say something, it doesn't make it happen. Beyond feeling guilt for causing the cancer, most siblings feel shame for their normal emotional responses to cancer like anger and jealousy.
Some children even feel guilt for being healthy.
Abandonment When parental attention revolves around the sick child, siblings may feel isolated and resentful. We went to get Daniel that evening, and he was very withdrawn. Then teacher turned out the lights, and you didn't come to get me. Then he burst into tears. In hindsight, one of us should have gone to bring him to the hospital to just sit with us.