My older cat being jealous

Question

My cat is a year old and I introduced him to our 3 month kitten who was a twin and it took 3 weeks before he started to play with them ,lay with them , groom with them even share their food together than our twin female became sick and died. Now our male twin is sick and our older cat keeps hissing at him while smacking him every chance he gets also hisses at our other cats even his owner why what does that mean?

Answer 1

You write

also hisses at our other cats even his owner

Whenever a pet suddenly changes his/her behavior, especially if it becomes aggressive or shies away from touch, it's time for a vet visit. This behavior can be a sign of an underlying medical problem or pain.

At 1 year of age he's probably due for a general vet checkup anyways, so let the vet look for any painful conditions like a bad tooth as well.

If that doesn't yield anything, I'd suspect the illness of the other cat triggered some stress symptoms in him. But that's more speculation than anything else.