Lady Edwina's mother, Her Grace, the Duchess of Westminster, is the Duke of Cambridge's godmother. I would imagine the plans were in place for some time for the Grosvenor family to attend the wedding, and it's very possible that Lady Kennard's death at age 91 was not a great shock to the family, including Lady Edwina.
I can recall receiving a bit of criticism when, after my father died on a Sunday, I worked that Monday through Wednesday. It could not have been helped, as I had responsibilities at work, and more still with the funeral on Friday. All went off without a hitch, but what I did that first week after my father's death had little to do with what I personally wished to do and more to do with what I was committed to do. It could be the same with Lady Edwina.