I’ve had two cats who have lived together for months. They always cuddled and loved each other and played amazing. I brought home two new kittens. One who’s been super aggressive towards them. The 4 month old kitten I’ve had hasn’t reacted to it. My 11 month old kitten is now aggressive to the 4 month old kittens she’s gotten along with great. Anything he goes near her she starts hissing aggressively and smacks him. She’s constantly hissing at him and I’m not sure why. It’s making me very upset since they’ve always gotten along so well and now she’s been super aggressive. Any help or answers would be great. Thank you