Complex sentence: He was very close to his cousin, Tomoko Oishi (whom he and his mother call "Tomo-chan"), daughter of his mother's elder sister, Ryomi, and was badly affected by her death.
Split into simpler sentences: He was very close to his cousin, Tomoko Oishi (whom he and his mother call "Tomo-chan"). She was the daughter of his mother's elder sister, Ryomi, and Yoichi was very negatively affected by her death.