My condolences to you on your losses and this boy's battle. My sister died after a 5-year battle with cancer. She was only 19. Your feelings are entirely normal.
There's not much great advice as everyone needs to deal with these things at their own pace and in their own way. But I'll try to give a few things.
1) as far as taking care of yourself, just remember that if you have any feelings of guilt over their loss that the only person that is hurting is yourself. it won't bring them back. if you have anger at god, the universe, etc. that will eventually spill over to the people in your life. so while it's understandable to feel that an implicit promise of religion, that the good will be treated well, has been broken, the anger is not a solution to your grief.
2) in taking care of others, often the best thing you can do is to just listen, hold their hand, and be there to hug them when they need it. there are no words that make it better for them. you can only re-affirm your love and care for them which will eventually carry them through.
best of luck to you. and I hope he defies the odds.