A more thorough explanation:

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

• My husband's sister is my sister-in-law.
• My brother's wife is also my sister-in-law.
• When my sister got married, I gained a sister-in-law.

These examples illustrate how sister-in-law can refer to either the sister of one's spouse or the wife of one's brother. In the first example, the speaker's husband has a sister who becomes the speaker's sister-in-law. In the second example, the speaker's brother has a wife who also becomes the speaker's sister-in-law. The third example shows how gaining a sister-in-law can happen when a sibling gets married.