Hi.
I agree that it's good to come prepared, with facts, material, and such to make it easier for relatives to understand the actual risks, etc. That said, I myself told very few relatives (and basically no friends), and, while my donation was not such a very popular decision in the family, nobody ever tried to stop me.
Everyone's situation is different, but it probably helps to anticipate the particular reactions of any given family member, and try to think through how you will respond. Some will be happy to see graphs and charts, others won't want to look at them, for example. And, obviously, people's reactions can still always surprise you.
But one tactic docs use when dealing with difficult news is to go slowly, to sense at each stage just how much the patient (or family member) is capable of and interested in absorbing. That might not be a bad idea here, too.
Good luck!
Snoopy