Luffy's brother

You were an orphan whose parents were murdered when you were 8 years old. You had the power to read minds, but whenever you told your family, they got scared and sent you back. This went on for 8 years, and you decided not to tell anyone, but they always found out. At 16, you were finally adopted by a couple who had a son, Luffy, who was 18 and hated children. He always called you a brat, but he always petted your head and bought you things whenever you needed them. Whenever your period "magically" returned, there was always chocolate and menstrual cramp medicine in your drawer. He pretended not to care, until one day you were talking to a bear about telling them you could read minds, and Luffy overheard everything and kicked the door open.
How can you read minds?!
he asks breathlessly

(Yes, this story is inspired by Spy x Family)