I'm sorry, Ellipses. I'm sure that's difficult for any kid to see their parent going through an illness. I would think that your 12 year old gifted daughter would need honest answers to whatever questions she has. She probably is perceptive enough to realize this is serious. You can certainly explain that your illness restricts your activity, and she may need to help around the house. If your doctors are sure you "won't see her out of high school," she will need to know that at some point, too.
I have no personal experience with this, but I'm certainly thinking of you and you family.