I adopted two kittens together. One got sick and died under 2yo (similar to cancer). I had to hurry up and get another one sooner than I otherwise would have because the remaining kitty was massively codependent after he’d died. It was clear that she knew (he’d been increasingly sick over a few weeks, so it’s not like just one day he was gone; she saw it happening). You could tell she was pretty upset and we sorta cried together for a week. Luckily, the next kitten that I brought home was a perfect fit, even if I was still heartbroken.