noun
brother-in-law (husband of one's spouse's sister)
Refers specifically to the husband of one's spouse's sister. This is a reciprocal relationship: two men married to sisters are 相婿 to each other.
彼は私の相婿で、妻の妹の夫です。
He is my brother-in-law, the husband of my wife's younger sister.
相婿同士で飲みに行った。
The two brothers-in-law (married to sisters) went out for drinks together.
Compound of 相 (ai, 'mutual') and 婿 (muko, 'son-in-law' or 'husband'), literally 'mutual son-in-law', reflecting the reciprocal relationship between men who marry sisters.