My sis purely adopt a cat ?

from a shelter he is 2 yrs old, he is super cute but he picks fights with the two other cats that she have had for awhile i told her he is just playing, but she swears he is extremely mean nearly it and its not at all a playful fight. we have have lots of cats in the past but this one i guess is really aggresive i dunno what to tell her
Probably anyone terriorial from being in a shelter with other cats. It is going to thieve time for it to relize that there is no need for competition if the cats get equal attention, food, and care.
Did she do a slow introduction or merely throw the new cat into the mix?
Any animal you adopt needs to meet your other pets first whether at all possible.
Do what the other person suggested.
Also, putting him in a separate room near only supervised "visitation" is a good idea.

Hopefully, they will work it out.
Answers:    Sounds approaching hes trying to prove its his territory. Make sure he has his own space, own bed, own toys, etc. Rub the toys around his scent glands on the side of his frontage, chin, neck, etc. so they know they're his. He could be bored too, maybe give him something next to catnip? Crazy wild cats love catnip, and keeps them busy for a while.
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