Complex sentence: The story of the film will revolve around Sakura Matō, the younger sister by blood to Rin Tohsaka, but when she was young, Sakura was sent to the Matō household as an adopted daughter.
Split into simpler sentences: The story of the film will revolve around Sakura Matō, the younger sister by blood to Rin Tohsaka. When she was young, Sakura was sent to the Matō household as an adopted daughter.