You are my brother, but I am not your brother. Who am I?

Why this works

This riddle plays on the relationships within a family and the use of gendered terms. When the speaker says, "You are my brother," they indicate a familial bond, but when they say, "I am not your brother," they reveal that they identify as female, hence they are a sister. The trick lies in the expectation that both people might share the same familial title, but the twist is that one can be a sibling without sharing the same gender.