A sister-in-law is the sister of one's spouse, the wife

Wife

A wife is a female partner in a marriage. The rights and obligations of the wife regarding her spouse and others, and her status in the community and in law, varies between cultures and has varied over time.-Origin and etymology:...

 of one's sibling, or sometimes the wife of one's spouse's sibling. Relative to a woman, the latter case (typically her husband's brother's wife; that is, the two women have married brothers) is occasionally called a co-sister-in-law, as they are each the sister-in-law of the others husband.

For example, if Jacob

Jacob

Jacob "heel" or "leg-puller"), also later known as Israel , as described in the Hebrew Bible, the Talmud, the New Testament and the Qur'an was the third patriarch of the Hebrew people with whom God made a covenant, and ancestor of the tribes of Israel, which were named after his descendants.In the...

 and Esau

Esau

Esau , in the Hebrew Bible, is the oldest son of Isaac. He is mentioned in the Book of Genesis, and by the minor prophets, Obadiah and Malachi. The New Testament later references him in the Book of Romans and the Book of Hebrews....

 are brothers, Leah

Leah

Leah , as described in the Hebrew Bible, is the first of the two concurrent wives of the Hebrew patriarch Jacob and mother of six of sons whose descendants became the Twelve Tribes of Israel, along with at least one daughter, Dinah. She is the daughter of Laban and the older sister of Rachel, whom...

 being Jacob's wife, and Adah being Esau's wife, then Adah is Jacob's sister-in-law, Leah is Esau's sister-in-law, and Adah and Leah are co-sisters-in-law to one another.