Germline pathogenic variants (PVs) in CBL is found in 15% of juvenile myelomonocytic leukemia (JMML) cases. Here we report siblings with CBL variations presenting a heterogenous JMML clinic and outcome. The index case was diagnosed at the age of seven, whereas the younger brother was 10 months old and the youngest was one month old. The hematopoietic stem cell transplantation was successful for the index and the youngest brother with event free survival, but the middle brother showed severe graft versus host disease. This study shows the heterogeneity of JMML and how the outcome might differ even within the family.