This is a tough issue and you can only do your best and learn from experience. Often stresses and tensions build up between the caregiver and the one receiving care, so it is important to have an open and ongoing communication about each other's frustrations. Be honest, but gentle and empathetic. Also, pick a time when the other person is feeling strong enough to face the issues. If your attempts at self-resolution fail, get professional counseling.
Preventative counseling is another approach that works well.
You know cancer and its treatment will cause stress in your life and between the two of you. Get counseling before the conflict happens, so you can better deal with defusing it. Ask your local cancer sources to refer you to a good psychologist who has experience with clients who have cancer. As caregiver, learn to be a good patient advocate. Mistakes will be made by health care professionals as you go through a lengthy treatment process.