Why isn't my cat eating or drinking?

I have a 7 year old dog and last year when I got a new chihuahua puppy, my older dog kind of did the same thing as yours. She kind of just strayed away from the whole family and seemed kind of sad all the time. It sounds arrogant and neglectant but we ignored her behaviour only slightly. When we would go to the park, on a walk, or just to the store real quick, we would take the older dog so she would feel needed and involved. half the time we would leave the puppy at home also so she would get too used to going with us everywhere and mourn when we weren’t with her. The older dog (who is a small 10 lb terrier) is also very protective. So when she would hear something or lol a mouse for instance, I would enphesize (sp?) her presence and encourage her to take action so she would feel needed. I don’t know how you could use these tips on your cat but there you have them (BTW: to this day, one year later exactly, she is still a little less cheerful AND about the eating thing! try to feed the cats seperately from each other. maybe opposite sides of the room or ( I havent’ had a cat in ages so I dont’ know if this sounds cruel or not but..) put the oldest cat in a room by her/his self with plenty of good-tasting food and water. see if she eats at all and if so, you can probably narrow down the possibility of her being ill.

hope I helped :)