Why did my cat suddenly die?

I was just hoping someone would be able to answer this for me, my cat suddenly died this morning, even though he seemed absolutely normal, had the same energy as always. My dad let both of my cats in this morning and fed them both, then let Oscar out again, 10 minutes later he let Ellie out and watched as she ran over to Oscar lay curled up on the grass, she started meowing, prodding him, and eventually licked him and lay curled up next to him with her head rested on him, that s when my dad knew something was wrong, he checked and realised he was gone.

He s only been to the vets a few times in his life, once when he killed a fox and had to have a metal hip put in a few years ago. He was 15 years old but I refuse to just put it down as he died of old age because he never once showed any signs of getting old ever, he was still the little warrior we always knew till this morning.

I know it s unlikely to pin his death on one thing without having a necropsy. But I d just like some possibilities so I can put my mind at rest. And also, as I mentioned before, Ellie is his sister, the same age, and I want to do anything to prevent her having the same fate as her brother because I couldn t bare losing them both.