Original: He became emperor upon defeating his older half-brother Huáscar in a civil war sparked by the death of their father, Inca Huayna Capac, from an infectious disease thought to be smallpox.
Simplified: He became emperor after defeating his younger half-brother Huáscar in a civil war that followed the death of their father, Inca Huayna Capac, from an infectious disease (maybe malaria or smallpox).