This pilot study examined changes in cancer-related post-traumatic stress symptoms (PTSS) across time for siblings of children with cancer. Siblings (N = 32; aged 8–18) completed a measure of anxiety, the Child PTSD Symptom Scale (CPSS), and the PTSD section of the Structured Clinical Interview for DSM-IV-TR (SCID) at twelve (SD = .9) and eighteen months (SD = 1.3) post-diagnosis. Moderate-to-severe PTSS was reported by 12 siblings (38%) at T1 and 7 (22%) at T2. Cluster analysis of PTSS data revealed five patterns: Few symptoms, stable across time (31%, n = 10); Mild symptoms, decreasing across time (16%, n = 5); Mild, stable symptoms (28%, n = 9); Moderate/severe symptoms, decreasing across time but remaining moderate (19%, n = 6); and Moderate/severe, stable symptoms (6%, n = 2). SCID data and anxiety scores distinguished siblings in the final two clusters from those with more favorable PTSS levels/trajectories. Additional research with larger samples is needed to validate these trajectories and examine factors that distinguish siblings with consistently elevated cancer-related PTSS from those with mild or significantly improving symptoms.