A more thorough explanation:

Definition: Brother-in-law refers to the brother of one's spouse or the husband of one's sister. It can also refer to the husband of one's spouse's sister. The plural form of brother-in-law is brothers-in-law.

• My sister's husband is my brother-in-law.
• My wife's brother is also my brother-in-law.
• When my husband's sister got married, her husband became my brother-in-law.

These examples illustrate how brother-in-law can refer to different relationships. In the first example, the speaker's sister's husband is their brother-in-law. In the second example, the speaker's wife's brother is their brother-in-law. In the third example, the speaker's spouse's sister's husband is their brother-in-law.