So sorry to read your story. It's hard to see why your in laws are so unsympathetic. It's incredible that facing a cancer diagnosis and treatment you are supposed to care about and apologise for disrupting their Christmas. Where are their priorities? How selfish and small minded are they? You don't need this tension while facing a very tough time. Try not to waste energy on them.
Like many people, my personal experience has been that a couple old friends were not very sympathetic or supportive about my diagnosis or treatment. One gave me the sense that talking about my cancer was poor form or bad manners, although her problems apparently were different and well worth discussing endlessly! Other people were fantastically supportive. People are so surprising, at least to me.
I suspect that cancer is a strong bright light that shows what people really are like and what your relationship with them really is. Sounds like your in laws wanted their Christmas and their happiness to be the most important thing, not your health and emotional state. Sounds like their lack of focus on you will continue. That is very sad for you and your husband. At least you know now not to expect much support from them during your treatment.
Focus on those who love you and support you. Try and rise above the pain of rejection. I personally found that really hard to do. Your relationship with them may never be the same as before your diagnosis as a result of this. That has been my experience. It's impossible to completely forgive a person who you thought would be there for you but who isn't.
Once you start treatment you are given steroids, and as a result you won't be so forgiving and appeasing either. During chemo I regularly called out bullying from my abusive father. Didn't stop him entirely but it was satisfying anyway.
Wishing you the the very best of luck with your treatment. Karen