Sexual connections between siblings-in-law are considered incestuous under Islamic and Jewish law, unless the spouse is no longer married. So yes, a man can marry his brother's wife. There are several reports of this happening in the history of Islam. For example, Ibn Ishaq records that Muhammad ordered his followers to divorce their wives so that he could marry his daughter in law Zaynab. After Zaynab died, he married another woman. The Prophet is reported to have said: "He who has sexual relations with a female slave must free her afterwards." This hadeeth was narrated by Abu Dawud, At-Tirmidhi and an-Nasaa'i. Al-Bukhari did not include it in his collection of Hadith stories. Islamic law prohibits marriage between first cousins. However, this rule is often ignored in the Middle East where polygamy is common. A Muslim man can marry up to four women at once (according to some scholars, three if they are from the same family). His first cousin once removed cannot be one of those women. In Islam, marriage is viewed as a contract, similar to a business agreement or a lease.