I had to travel to Plymouth to pick up a mother cat and kittens ten years ago they were left with no food and the person I was picking up was about to get evicted from their home.
Ruby passed away suddenly, she was a Bengal cat all the cats noticed and acted differently, the day this happened Sandy insisted on getting under the covers curling up against me and kneading me like a little kitten does to her mother.
For the first few weeks she continued to do this, Possum her sister went very reserved I had paid attention to both in different ways.
I kept the routine the same within the home cats can show signs of grief as in loss of appetite, crying, change in sleep patterns, seeking extra attention and generally sad or searching for the other animal or if they lose a loved person they may sit in the place where that person sits, this can be very stressful for them especially if they become homeless or re homed.
Cats are very loyal and connected they have an interesting social system like humans they will make friends with some and others they will not get on with so replacing an animal is not that simple since they have built relationships with the living soul having a special bond, this is the reason they may become withdrawn and depressed demanding attention.
Cats are very spiritual and live-in harmony this is why a cat may become unsettled if they feel any tension, sometimes it is helpful for them to say goodbye to the body after a furry friend that has died, when they have examined the body allowing them to accept their mate has gone, with a cry or ignore all together which is normal.
Cats are very sensitive and will notice spirits I believe they may stare at something or act differently, when I lost a cat called Ginger Megs there was no other cats in the room, yet I felt the cat jump on the bed and no cat was there, I believe they stay around for a short time and the living cats can pick this up, which helps them accept the loss in some cases depending on how close the animals were the others, sometimes die within months or short time afterwards.
It is important that these souls get love and attention since security is what gets them through, re homing an animal can be difficult although they accept their new surroundings, they do not forget the connections with people and other animals, when the person they love comes into the room they will show the same affection and love as if they are still in their lives, their chosen one.