That sucks, man. Id like to say sorry or I can relate or something, but I hate sounding patronizing. The only relative Ive had die in my lifetime was my grandparents dad, and I was 5 I think. So, like you, I didnt know what mortality was.
Two years ago, my best friend died, however. I didnt handle that well, but Im ok now.
A message to everyone, cherish every moment you spend with your loved ones...