At the beginning of this century there was only one plural for this word – mothers in law. The word “mother in laws” was considered unacceptable. As a result, we had “fathers in law”, “brothers in law”, “sisters in law”, etc. Of late, native speakers of English have started saying, “mother in laws”, father in laws”, “brother in laws”, etc. So the plural of “mother in law” could be either “mothers in law” or “mother in laws”.
The Hindu – KYE, September 04, 2001
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