The Irregular at Magic High School, Vol. 8 (light novel‪)‬

Looking at Miyuki and Tatsuya now, it might be hard to imagine them as anything other than loving siblings. But it wasn't always this way...
Three years ago, Miyuki was always uncomfortable around her older brother. The rest of their family treated him no better than a lowly servant, even though he was the perfect Guardian, watching over Miyuki while she lived a normal middle school life. But what really bothered her was that he never showed any emotions or thoughts of his own.
However, when danger comes calling during a fateful trip to Okinawa, their relationship as brother and sister will change forever...