Complex sentence: Tsubasa wears the mark of Orochi over his heart; he is Sōma's biological older brother, leaving him (as a young child) with the Ōgami family when he became a fugitive.
Split into simpler sentences: Tsubasa wears the mark of Orochi over his heart. He is Sōma's biological older brother, leaving him (as a young child) with the Ōgami family when he became a fugitive.