I am so sorry to hear about your mother-in-law. Cancer is a very difficult thing to understand. My dad was just diagnosed about a month ago with advanced cancer and we don't know how long he has left. He is only 63. It's been hard both my sister and I because we're only 25 and 23. Cancer is one of those things that we can't make sense of. I know this won't really be any consolation but atleast she didn't have much pain or suffering. But I also know the shock and disbelief of not knowing, and regretting not finding the cancer sooner. Try not to be too hard on yourself, and I also find that talking to friends or talking to others to help with the emotions a bit. And I always think it's okay to cry! I cried so much in the first few weeks of finding out about my dad. I hope I can come to a point of accepting this. 

If you ever need to talk, i'm always here. Sending my thoughts to you at this difficult time. xx

I hope some of this advice helped a little bit, as I know there's probably nothing anyone can say to make you feel better (atleast knowing from my experience :(