My cat won't eat, could it be that he's sad?

Don't let your sister bring her cats anymore. You're living in your cat's territory, and you just ruined his territory for a while. Now he's got to get rid of all that stinky other-cat smell that has marked his sign post throughout his territory. It's hard work maintaining proper territory limits. You're part of that territory and you just dissed the ever-loving bejezuz out of your cat. In human terms, it would be like Mitt Romney showing up at the White House and moving in for three days. Worse yet, you confined Barrack Obama into a smaller space so the intruder could stay.

Suck it up. Make sure he is re-established thoroughly as king of your domain. You are, and have always been, merely a willing subject. (Lots and lots of doting until he gets over it, and then back to the usual amount of doting. LOL)

Wash thoroughly with bleach, as many spots as you can think of that the interlopers touched. You can't smell the stink, but your cat is smelling nothing but the interlopers. Like blood at a crime scene, you cannot wash it all away, but your cat will make sure to remark the sign post. (Just in case you don't know what sign posts are, they're the places animals rub their scent glands against to make sure all other animals know they were there and it's their territory. Rather like bulletin boards and "Do Not Disturb" signs, and yes, your legs are probably a sign post, if you usually treat him right. LOL)

Our cats feel dissed every time we go away on a vacation, because we don't take them. They let us know it for days afterwards. They will find a spot on the floor in front of us (and they never sit on the floor any other time), and make sure their back faces us for days. (There are six of us, yet they try to ace their backs to all six of us at once. LOL) We have to dote, until they pardon us. Fortunately, food is never an issue.

I'm thinking your cat might hate her food dish now, because the interlopers used it/got close to it/defiled it. I wouldn't go with wet cat food, since the only thing that teaches a cat is how to be finicky on what type of food to eat. (Dad made the mistake of feeding one of our old cats shrimp, back when he was dating Mom. The cat never went back to dry cat food after that, and demanded table scraps. LOL) A new food dish, water dish and pad (after thoroughly cleaning underneath with bleach) might help.

And, I'm guessing you might think I'm taking this too far, but maybe not. After all, you think a cat is sad. In my mind, our cats have two emotions--content or PlSSED! They really do have a territorial need built into them, and your sister's cat ruined your cat's life for tree whole days. I suspect he will forgive you, eventually. ;)

No he is not sad. He hated having those other cats there.

Cats cannot go longer than 3 days without eating or the chemical changes in their bodies affects the liver and that really makes them sick and feel bad which makes them not eat. Bad loop because they in effect become anorexic and if you can't get them to eat they must have a feeding tube surgically implanted and you have to feed special food through the tube four times a day. So you need to get your cat to eat anything to go back on food.

Try a good brand of canned cat food. If he is not sick, this should do it.

There is a possibility he or she could be sad. He or she just could have a tummy upset, depends what he has done in his life really my cat has done that.

I agree, try wet food. And give it lots of love, w/o smothering it though.

Take him to an animal doctor

Try some wet food (:

Try petting him, or giving him space.