Family & Friends of Cancer Patients Support Group
This community is dedicated to parents, siblings, grandparents, other relatives, friends and caretakers of cancer patients. It's difficult to find a safe place to talk about caring for a loved one with cancer, and so this group serves as an open forum to foster positivity and support for those going through this tough time.
Your first responsibility is to yourself. If you feel the need to talk things through, seek out counselling. There are helplines, or if you need something more you could arrange one-to-one sessions through your doctor, or find a group session of family and friends of cancer sufferers, you could look at cancer charities for this info.
It is totally up to you what you do. Nobody would blame you if you decided not to have anything to do with your sister. I don't know your history or what she has done, but I am sure you have reasons for separating yourself from her and her getting cancer does not change those reasons, or whatever she has done.
It could be that you feel the need to reach out. You could do this in or you you could write a card or letter. But personally I think that you need to get your feelings straight in your own head first.
Personally I would tell your spouse about it, or at least tell someone close to you, it sounds like keeping it all inside is hurting you. But tell him you need his support not judgement, that you are aware of his opinion on your sister and not interested in hearing it again, you just need someone to listen and sympathize.