M.J.
First, let me offer my condolences about your sister. That's very tragic and I'm so sorry about it all.
In your sister's situation, she should have changed oncologists. Kaiser is so big on service and they survey regularly about experiences with doctors. It's super easy to change doctors if you want to. Doesn't matter which health insurance you have, you should never settle for a bad doctor. So, whether you go with Kaiser or another provider, keep that in mind. It's your health and you have to be your own advocate. I've been with Kaiser most of my life and can tell you the absolute most incompetent doctors I ever found were during the three years I was out of Kaiser. I was shocked at how poor people are treated outside Kaiser. It was so bad I made my husband talk to his employer about adding Kaiser so we could switch back, which they did.
A local oncologist with Kaiser is the one who created the breast cancer awareness postage stamp and is up for a presidential honor this year. He's also known for being a top-rated oncologist. So, not all Kaiser oncologists are lousy.
My dad had prostate cancer and an excellent experience with Kaiser. Because they're so vigilant about preventative medicine, it was caught very early. He got great treatment from a skilled surgeon and had no bad side effects that can happen when this surgery isn't done well (my uncle and FIL outside of Kaiser both had problems -- my uncle's was quite serious -- after prostate cancer surgery). His doctors and nurses were awesome. They've been great about monitoring things since then.
I think you can have good and bad experiences with any health plan. The key is speaking up when you're not happy and insisting on the care you deserve.
ETA: Get in for that mammogram now. Kaiser will do them early if you are high risk. I have friends who are in that category and Kaiser started mammograms with one of my friends in her 20s. It's always better to find out early you have cancer than to hold off. I know with my dad's cancer, the survival rate is very high when caught early but it's quite deadly when it's not. Get in now so you can be around for your kids.