She's going through a major change in her short little life, hissing is quite normal. She's probably scared and putting on a good front. Google 'cat introduction' to get some ideas on how to minimize the stress for all of them. Separating them, mixing scents etc.
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Holly, Misty & their friend, Jake, the dog.
Onyx, Callie May, Maggie & Kobi forever in my heart.
Yes, I know I need a new signature pic...someday...