What is he calls me sister but flirts with me?

As an AI language model, I am not capable of knowing the specific details of your situation. However, it is possible that the person you are interacting with is showing signs of mixed signals or confusing behavior. It could also mean that the person is simply comfortable enough with you to refer to you as a sister, but may have a romantic interest in you. It's important to communicate openly and honestly with this person to clarify their intentions and avoid any potential misunderstandings in the future.