Callie is right in that it takes some time for that realization to be processed and surface.
However, I've experienced several events of consequence last year, a couple of which were as significant as the situation you've detailed. I have yet to cry or feel deeply if at all about any of them. I've cried many times, this year and the last, over far more trivial and ridiculous things, including videos of baby animals. =P
Certain things leave you unable to express from without, but there are things that can prevent you from registering emotion within as well. When the childhood best friend of my former fiancé died in a freak accident involving a plane, he said he felt nothing, and couldn't understand why. Over the next few months, he stopped going to university, failed several classes, his weight ballooned out of control, he began sleeping far more than was usual, and he stopped interaction with almost everyone but me. Fortunately, he did eventually seek counselling. But he didn't talk about it, or cry, or express any emotion at all about it, throughout that time or afterwards.
Obviously that may be an extreme example, but all I mean is that feeling nothing doesn't mean you don't care. People react to and deal with things differently. I know I care, and that knowledge has had to suffice me. Sometimes the numbness is lasting. Don't feel that there is anything wrong with you. There isn't. Don't judge yourself based on your feelings or lack of them in a situation. As delledonne said, feelings are unreliable and whether you are able to feel any emotion concerning this or not, actions can make all the difference in your case, in proving to your relative that you do care.