I have 2 cats that I've had for about 2.5 years since they were babies. In May I rescued and started fostering 2 kittens. Recently we started letting them roam about our apartment while supervised to socialize with our 2 resident cats. When they're not roaming about we put them in the guest room with the door closed with a litter box and toys. One of my resident cats (Petey) has adjusted fine and has no issue with the little ones. My other resident cat (Penny) is having trouble adjusting, hissing/growling at them, hiding and even lashing out occasionally at Petey. Is there anything I can do to help her calm down and be comfortable with the kittens? We're hoping they'll get adopted soon but if they don't they'll be here for a while and I don't want Penny being in a constant state of stress.