why did Muhammad his cousin Zainab? was she Muhammad's sister?

 Muhammad married Zaynab bint Jahsh, who was not his sister but his cousin and the daughter of his aunt. Zaynab was previously married to Muhammad's adopted son, Zayd ibn Harithah, but they divorced. Muhammad then married Zaynab as soon as her waiting-period from her divorce was complete, on 27 March 627. He went into her house when she did not expect him and without knocking. Zaynab asked him: "Is it going to be like this, without any witnesses or trustee (wali) for our union?" Muhammad replied: "Allah is the witness and Gabriel is the trustee."

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what is the historical context of Muhammad's marriage to Zainab?

The historical context of Muhammad's marriage to Zainab bint Jahsh is rooted in the social norms and customs of pre-Islamic Arabia. Zainab was previously married to Muhammad's adopted son, Zayd ibn Harithah, but they divorced. In that society, it was considered taboo for a man to marry his adopted son's ex-wife.

 Muhammad's marriage to Zainab was meant to abolish this taboo and establish a new social order based on Islamic principles. The marriage was also a way to demonstrate that all Muslims, regardless of their social status or lineage, were equal in the eyes of Allah.

 Muhammad's marriage to Zainab was not without controversy, and some of his followers criticized him for marrying his cousin. However, the marriage was ultimately accepted by the Muslim community and is considered an important event in the early history of Islam.