Nephew in the context of "Sibling-in-law"

An avunculate marriage (or uncle/aunt-niece/nephew marriage) is a marriage with a parent's sibling or with one's sibling's child—i.e., an uncle or aunt marrying their niece or nephew. Such a marriage may occur between biological (consanguine) relatives or between persons related by marriage (affinity). In some countries, avunculate marriages are prohibited by law, while in others marriages between such biological relatives are both legal and common, though now far less common.

If the partners in an avunculate marriage are biologically related, they normally have the same genetic relationship as half-siblings, or a grandparent and grandchild—that is they share approximately 25% of their genetic material. (They are therefore more closely related than partners in a marriage between first cousins or between granduncle/grandaunt and grandniece/grandnephew, in which on average the members share 12.5% of inherited genetic material, but less than that of a marriage between, for instance, cousin-siblings, in which the partners share 37.5% of their inherited genetic material.)

ʿAbd Allāh ibn ʿAbbās (Arabic: عَبْد ٱللَّٰه ٱبْن عَبَّاس; c. 619 – 687 CE), also known as Ibn ʿAbbās, was one of the cousins of the prophet Muhammad. He is considered to be the greatest mufassir of the Qur'an.

He was the son of Abbas ibn Abd al-Muttalib, an uncle of Muhammad, and a nephew of Maymunah bint al-Harith, who later became Muhammad's wife. During the early struggles for the caliphate he supported Ali, and was made governor of Basra. He withdrew to Mecca shortly afterwards. During the reign of Mu'awiya I he lived in Hejaz and often travelled to Damascus. After Mu'awiya I died in 680 CE he migrated to At-Ta'if, where he is resting from around 687 CE.