This is tough. I suppose she thinks it is contagious (of course it isn't) and/or she thinks you are about to die from it (No!). It sounds like CML is one of those cancers that can be treated and the person with it can live a long time. In fact, there was a recent study showing no difference in mortality for CML than for the general population. There have been tremendous advances in treating this since 2000.
It strikes me that knowledge is what she needs. Maybe she would agree to talk with your oncologist. Maybe there is a counselor who can help. Perhaps a minister?