Well, my two cats are brothers, and grew up together, and still attacked each other until we got them neutered. Now sometimes they cuddle together, and sometimes they pounce on each other when unsuspecting and one goes meowing and running out of the room. For a while we had an upstairs cat and a downstairs cat and they had each better remember which was which. Cats are strange animals. The best thing going for ours (beyond neutering) is that our dog steps in and referees. She won't put up with any fighting and puts everyone in their place in a microsecond. Probably not all that helpful. I hope yours decide to like each other after they get used to each other. Evie will probably want to be friends, and Ellie will probably want to keep all your attention to herself for a while, but I bet she'll get over it. Kittens are cute, even to other cats.