You know you have to introduce cats like you do chickens. SLOWLY. Locked room for the newcomer - feed by the door - they get used to sniffing each other under the door when they eat. Then supervised. Just because
@CrazyChookChookLady can take any animal and plop it right in the middle of her family and they all love each other immediately, that is not normal!
Two of my cats still don't speak to each other and they have been together nearly 10 years, but way back a decade ago, one of them was a newcomer and usurper of affection and has never been forgiven.