A brother-in-law's one's sibling's husband, or one's spouse's brother. One's spouse's sibling's husband isn't considered a brother-in-law. The plural of this term's "brothers-in-law". Example: Joe and Brenda are married. Brenda has a sister, Irene, and so Joe's Irene's brother-in-law. Irene will marry Jason, who'll then become Brenda's brother-in-law. Joe and Jason are also brothers-in-law.… (More on Brother-in-law)