I'm very sorry to learn of your brother-in-law passing so suddenly. Your sister must be devastated. Losing a loved one is hard enough when its anticipated but out of the blue is another story.

Assuming that your sister and her husband are somewhere in our age bracket, it's little consolation but from another perspective it can be thought of as a blessing. Their last day was spent together having a good time. His cemetery comment can be interpreted as him being in a good mood. He was home, in his own bed and apparently quietly shut down. The kind of passing most of us hope for although shocking to the survivor.

I hope your sister's grief doesn't overcome her and that she comes to think of his passing in a similar light.