A brother-in-law (plural brothers-in-law) is the brother of one's spouse or the husband of one's sibling. A brother-in-law can also be a husband of a sister-in-law.
A sister-in-law (plural sisters-in-law) is the sister of one's spouse, the wife 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 other's husband.
For example, if Jacob and Esau are brothers, Leah 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.