I am soooo sorry you and your families had a young, precious little girl ripped away by an terrible illness. Just reading has made me turn into a big cry baby. As I am a mom of 3 kids myself.... 7 mo, 5yrs and 7yrs... I too fear of losing my little ones and etc. My shoulder you can cry on if you wish. It is okay to cry, it is okay to beat up the pillows and bed to let out your anger, it is okay to scream at the top of the hill or empty land....
She is now not suffering anything and looking over her wonderful gifted families that has showed her nothing but, love, respect, a wonderful life, caring, laughs, silly moments and so much more wonderful things.
as for explaining to your son, you can probably say this, "She was really sick, God didn't want her to suffer so he came to get her put her in a better place where we all will meet up again. She will be there waiting for us to play some more when time is right."