My brother has adult chronic leukemia. what should i tell him?

Treatable: The typical chronic leukemias are CML or cll. Nowadays CML has become a chronic disease for most adults and most patients are expected to do very well with oral meds alone. Many cll patients are just observed in the beginning. There are many effective treatment options available too.

See your doctor: Are you your brother's doctor? It is a subject he should take up with his doctor. Leukemia is a serious though often treatable illness and he should see his doctor asap.