What is wrong with my sister this not normal?

I seriously think there is something wrong with my sister she is 15 and im 17 i think this because we would be talking about something like ill ask her what she wants to eat for dinner and she responds "omg i got a scab from riding my bike in the park last summer. Or i would try to seriously talk to her and she starts telling me all theese strange things like you remind me of a clown or why are weat things so thin. Me and my friends were at lunch having a conversastion and out of nowhere she starts talking about things like i had a pet rock but spark ate it. I asked her who spark was and she told us her imaginary dog. So to keep her quiet j give her some kids fruit snacks because she likes to play with them. So i woke up in the middle of the night and she was sittjng on the floor screaming i asked her what was wrong and she td me she wanted her teddy bear so i asked her where was it and she told me she didnt know that she lost it last summer in the park. What is wrong with her