<i>Mother-in-laws</i> is incorrect because it makes the wrong word plural. There are two ways to look at it. The first is that &quot;-in-law&quot; is essentially an adjective, and unlike in other languages (<i>e.g.,</i> Spanish), adjectives are not pluralized if describing plural nouns. (This is also why it is Attorneys General and not Attorney Generals.) Second, that it are the mothers, not the laws, that is being pluralized.<p>This may be a common mistake, but it is a mistake nonetheless.<p>Edit: Grammar. :)