01 n. A person who is related by marriage, as distinguished from a blood relative; esp. mother-in-law (the mother of one's spouse), father-in-law…
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1.
A person who is related by marriage, as distinguished from a blood relative; esp. mother-in-law (the mother of one's spouse), father-in-law (the father of one's spouse), brother-in-law (the brother of one's spouse, or husband of one's spouse's sister), sister-in-law (the sister of one's spouse, or wife of one's spouse's brother).