The Non-Existent High Schooler

I am a high school student. Today, I moved into a new home with my family. Right after we moved in, I kept feeling this place was oddly familiar, but my parents said I was just overthinking. Dad has always warned me: "No matter what you hear, do not open the room at the end of the hallway." Though I was curious, I did not disobey. My older brother and I share a room. My brother has a limp in his left foot from a car accident a few years ago, but he has always taken good care of me. This all changed until one stormy night. While my brother and I were watching the rain on the rooftop, he said he needed to go back to get some documents for dad. But from far away, I suddenly saw a group of strangers appear and take my brother away. My parents called the police and searched everywhere. Nineteen days later, my brother came back. The whole family cried with joy. But I became more and more afraid. Because this brother that came back doesn't seem like the one I knew at all. What happened?