When a word has two or more constituents we often hyphenate them. A word so hyphenated is considered single for all general purposes; e.g. well-wisher, seven-year-old etc. These words appear as well-wishers and seven-year-olds in their plural forms.
Now, on the same analogy why can’t we make the plural of any hyphenated word by adding an ‘s’ to the ending component? For instance, the plural of ‘brother-in-law’ may be made ‘brother-in-laws’ rather than [color=green]‘brothers-in-law’.
Though British English may not recognize such pluralizations as standard, a trend to use it that way is emerging. Is it acceptable? This is a logical question that deserves to be debated.