Complex sentence: Having died six years after his older sister Alice, he was the second of Queen Victoria's children to die; his mother would outlive him by 17 years, by which time she had already outlived another child, Alfred.
Split into simpler sentences: Having died six years after his older sister Alice, he was the second of Queen Victoria's children to die. His mother outlived him by 17 years, by which time she had outlived a third child, Alfred.