A.,
I see some wonderful answers here. I am inclined in the same direction as KansasMom (wonderful post) which was then agreed with by Donna C.
I too, tell my boys the answers to their questions "like it is." - To a degree.
My boys are 9 and 6, but the 6 year old only knows slightly fewer things than the 9 year old. They view themselves as peers. So, when answering the 9 year old's questions when both are present, I always temper the truth with what the 6 year old can process. Sometimes, I tell the 9 year old extra truths as an aside. My favorite example is the question of what makes womens' "boobs" need to be bigger than mens', which was followed quickly with "well, if a man DOES have big boobs, can he?....." Which led to: "no, he cannot because having the baby removed from mommy is what makes the milk know to arrive...."
I say all this to say - I sort of wing it. I always start out with a basic truth - I would never want to get "called" on something later and have to undo it. But, I start with something general, and feel my way through whatever questions follow.
I think a major thing that may be troubling your daughter is that she has a solid answer as to the cause of death for YOUR parents but the deaths of her father's parents remain a mystery. And we all know what kiddos can do with a mystery.
You could always go down the path, as was suggested, of "Well, Grandpa was very sick already. Grandma got hurt so badly that she couldn't recover from her injuries and then, so too, did he. We can be happy for them that neither had to stay here on earth without the other." (thereby avoiding making a statement about heaven, if the events trouble you in that fashion)
You've got a tough one, and it sent chills down my arms and legs to read, so, I know it's not easy. And I wish you well.
B.