Shikkaku kara Hajimeru Nariagari Madoushidou!: Jumon Kaihatsu Tokidoki Senki

The Raythefts are an old, minor noble house defined by magical talent and martial service. When six-year-old Arcus Raytheft, firstborn son to the family, proves disappointingly inept at magic, he is stripped of his inheritance and written off by his parents. His adoptive sister Lecia still adores him, but as the new heir, she is forbidden to interact with anyone who might drag her down. But when one inheritance is lost, fate arranges another: Arcus remembers another life, in a world where science prevails and magic belongs to the realm of fiction. Suddenly endowed with the life experience of a grown man and pressed to find a purpose as his family turns against him, Arcus resolves to find a way to break the laws of magic and Raytheft tradition over his knee! (Source: J-Novel Club)