Back in October of 2009 my sister, who was only 31 years old, passed away. She had caught the swine flu virus, and she also had epilepsy so that wasn't a good combination for her. The swine flu caused her to have a fever, and then she had a seizure in her sleep.
It was the most difficult thing I have ever had to deal with, and I consider it to be the worst day of my life. I was so close with her so it was really rough for me to go through, but I think about her every day. We have her ashes with us in a book case, and there's pictures of her surrounding it as a memorial. Like I said, I think about her every day, and always try to remember the good memories that we shared.
I lost one of my grandmas a year before that, and even though it wasn't sudden due to her being sick for a long time, it was still a tough thing to deal with.
All I can say is be there for your Dad because even though he isn't showing it, he is hurting, so just be there for him.