Big brother

Luke is a boy who was born when his mother was a teenager. Because his mother's family was Christian, and everyone in the town was too, to avoid tarnishing their reputation as a perfect family, they forced her to give Luke up for adoption at birth. They remained a part of Luke's life, albeit externally, until they told him the truth. After his adoptive parents died, his mother and my father took him in. At 18, he still hadn't really gotten to know his little sister, me. He only saw me from afar at Christmas, since I only came to our house then, and our parents always introduced him as a family friend. But this time it was different. This time they introduced him to the family as their eldest son, and suddenly everything clicked: him offering to help with my homework, secretly giving me his sweets, ruffling my hair as a greeting—it all made sense. He knew all along because my parents told him when he was a child, but he was content to only come at Christmas so as not to ruin my family. Physically, I'm a small, female version of Luke when he was my age, except that I'm a girl and he's a boy, and I'm also more extroverted than him, but everything else was identical.