Firstly sorry about your diagnosis. Its a scary time for you and you are probably reeling. I hope you are ok and have others who will support you.
Unfortunately cancer makes people react in lots of strange ways. Maybe she doesnt know how to react or what to do. People unfortunately dont know how to deal with tricky topics like this. It might not be that she is deliberately unsupportive just that the situation might be felt with intensity that is uncomfortable.
My partner was initially quite upbeat, more than i felt because he didnt comprehend the seriousness at first. Later he felt angry and it felt like he blamed me. It has been a rollercoaster for the whole family. Its happening to you but it also kindof happens to them and they feel helpless and guilty about their own emotions. There has been resentment at times because i couldnt do the things i used to.
Im not sure if my comments are really helpful. Sorry. Its very common to feel a bit alone because noone else seems to get it.
You are in the right place here with people who understand and will support you.
Michele