A brother-in-law is one's sibling's husband, or one's spouse's brother. If broken into two degrees of separation, it's called a co-brother-in-law, such as one's spouse's sibling's husband. The plural of this term is "brothers-in-law". Examples: (Opposite-sex marriage): Joe and Brenda are married. Brenda has a sister, Irene, and so Joe is Irene's brother-in-law. Irene will marry Jason, who will then become Brenda's brother-in-law. Joe and Jason are also brothers-in-law.(Same-sex marriage): Richar… (More on Brother-in-law)