I would first like to say how sorry I am for your loss.
Without a necropcy there is no way to know what killed her, and with another grey in the house I would say you need to get one done. It sounds to me like it was some kind of poisoning because of just how sudden it was. I would think it was from vapors if there was cooking in the house right before her death. She may have been a little more sesitive to the fumes than your 5 year old, but this is just a guess...nothing more.
I really hope you change your mind on the necropcy, because the cost of it is worth knowing how to protect the life of your other grey.