Final answer:
Umar, also known as Omar, the second Caliph of the Islamic Caliphate, was assassinated in 644. His death led to a contested succession, resulting in Uthman ibn Affan becoming the third Caliph. The exact details about Omar's brother-in-law and sister and the number of Hafiz who died in battle are not available in the provided material.
Step-by-step explanation:
The question pertains to the historical narrative of Omar, also known as Umar ibn al-Khattab, the second Caliph of the Islamic Caliphate following Abu Bakr, and the developments after his assassination. Umar was assassinated by a Persian slave in 644 during a pilgrimage to Mecca, which led to a contentious succession, with Uthman ibn Affan becoming the third Caliph, rather than Ali ibn Abi Talib, who was another strong contender as Muhammad's son-in-law. The history of this period is marked by the continued military expeditions under Uthman's rule and the internal disputes within the Muslim community about succession and leadership after Muhammad's death.
Regarding the second part of the question, the specific information about Omar's brother-in-law and sister and how many Hafiz died in battle is not provided in the reference material. Therefore, without additional context or historical records, it is not possible to accurately answer this part of the question.