What is recently my sister has been acting strangely?

As an AI language model, I do not have personal information about your sister. However, if someone close to you is exhibiting strange behavior, it could be a sign of a number of things. It could be due to stress or anxiety, a medical condition, drug or alcohol use, or even a mental health issue such as depression or bipolar disorder. It's important to speak to your sister and express your concern, and encourage her to seek help if necessary. It's also important to remember to approach the situation with compassion and understanding, and without judgement.