How you feel is not a matter of choice. How you act is. You can choose a level of involvement that does not get too uncomfortable for you. You have the option of writing off your parents and having nothing to do with them. I'm not hearing that you are inclined to go that far. Ask yourself what your folks would likely do, if you had been the one diagnosed with cancer.

Relations with family are complicated indeed. You have no obligation to put yourself in a position of your mom using you for target practice. rThe past is passed. Maybe offer the level of support you would show to an aunt.