First, don't assume that her comments were a shot at you. Many women feel the same way she does and that's OK. Maybe they were a shot at you, maybe they weren't. It's water under the bridge at this point.
I think if I were in your shoes I would have a very brief heart-to-heart with her in which you say something like "I know that we don't see eye to eye on everything and we have our history, but I love you more than you know and I'm so sorry that you're going through this and if you let me help you, I will do anything you need. If it would help you more for me to *not* be involved, I understand, but I'm here for you, whatever you need, whenever you need it."
If I were diagnosed with breast cancer and facing surgery, radiation and chemo my biggest concern would be for my kids and running my home, so I think you should offer your help there. Can you help take care of her kids while she is in the hospital and going through treatments? Do they need someone to be there in the morning to get them off to school/daycare while she rests? Do they have activities and sports that they need rides to? How about meals, laundry, grocery shopping, cleaning the house, mowing the lawn, etc.? Are you good at coordinating people? There is a great website called lotsahelpinghands where you can set up a site and give access to those who can and want to help. You can schedule who is bringing dinner, who will give her rides to chemo, who will baby-sit, who can do chores and run errands, etc.
Even if your mom is jumping in to help take over the day to day tasks, I know mine would, it's a lot for one person to take on. So volunteer for something that's a pain to do to give whoever the main helper will be a break. I don't think anyone would argue with having the grocery shopping or laundry magically done, right?
Sorry that your sister - and your family - are going through this. Maybe the upside of this crisis will be that you can all get past your differences and have stronger relationships with each other.