Grammar Friday

I've found lately that I'm in need of a place to vent about incorrect grammar. I thought posting here would be better than becoming "that person" in the comment sections of other websites and blogs. (I do possess some semblance of manners, so I'm well aware that it's rude to correct others). And so, here is the first installment of Grammar Friday.

The plural of mother-in-law (or any other "in-law") is motherS-in-law, not mother-in-laws. In the plural, there is more than one person, not more than one law.