I am so sorry about your little female. It's always hard to lose a cat, but worse when they are young and even worse when it happens so suddenly like this. Cardiomyopathy (disease of the heart) would certainly be at the top of my list; it is responsible for many sudden deaths and often gives no signs before the fact. Heartworms could be possible although we often see signs before the fact. I've never had a cat be scared to death so I wouldn't consider that.
I do think heart disease is most likely or possibly a congenital disease that also was not showing any symptoms. I'm sorry I can't give you an answer but it sounds like you took wonderful care of her and there is likely nothing you could have done to prevent this. Keep an eye on her brother and continue your preventative care with him. I would remind your veterinarian to listen carefully to his heart at each visit and if a murmur is heard an ultrasound would rule out cardiomyopathy. Please again accept our condolences on your kitten.
More...