If a person believes they've changed into an animal, they suffer from a disorder called zoanthropy. You might worry your sister has zoanthropy if she only speaks in meows — but she's probably just pretending to be a cat.
Another term for zoanthropy is "clinical lycanthropy." Whatever it's called, it's an extremely rare mental illness in which a person sincerely believes they're in the process of becoming an animal, or that they've already become one. Zoanthropy is unusual but very serious, usually showing up along with psychosis, when someone loses all touch with reality. The word comes from Greek roots zoion, "animal," and anthrōpos, "man."