More hugs for you.
I don't think that an instant response is necessary or expected. Say whatever you would say if you had just received the news. send condolences, love etc.
I personally would probably not mention your DH as however sympathetic your stepmom is, her focus will be on her father, not your husband, and if she is inclined to be touchy then she could interpret it as you putting your concerns over hers.
If, and only if, she queries why you didn't respond faster, you can then explain that DH was very ill and had a crisis, and that you responded as soon as you were able to. (which is true)