Dealing with cancer is no different than any other life (or death) experience.
Going to the supermarket
Attending a kid's birthday party
Getting your tax return auditedPeople die at the supermarket, at birthday parties & at the IRS. Difference is, you don't expect it. It would come as a "surprise". Even though we know death could come to us at any time, we never want to see it coming.
Of course, another difference about the cancer experience is the emotional instability that engulfs everyone involved that makes it all the more frightening and uncertain. In reality, many people respond to cancer just like your sister-in-law. My suggestion is, just carry a rolled up newspaper in your purse at all times. lol
I hope your father in law is at peace and tranquil. If he isn't, that's too bad since his prognosis is not great.
I hope you and your husband are at peace with yourselves and each other, too. When you are upset and feeling dread, its easy to lose focus and concentration. On the flip side, when you're calm and at peace, you make better decisions and become confident. The calm state of mind opens the doors to more positive and successful outcomes...Yes, even with cancer, there are opportunities for inspiring and unforgettable experiences. Moments of great love and caring that can never be experienced if not for occasions such as these.
Don't expect the worse. Expect the best and except less, if required.
PS: I've buried two family members this last year. One, was my father. I bought eggs and tomatoes at the supermarket, this morning. I survived, both, just fine. Good luck and please drive safely.