WHile we're playing with apostrophes (see what I did there?) "mothers in law" is the plural of "mother in law", but the possessive of "mother in law" is "mother in law's". The plural possessive (but if you have two mothers in law somultaneously, you are likely, in the US anyway, to be behind bars)of "mothers in law" is "mothers in law's". Incidentally in this, as with "fathers in law", hyphenation is I believe, optional. I usually do not do it, unless I feel it makes the sentence in which the phrase is used, more readable.
"There's only three tempos: slow, medium and fast. When you get between in the cracks, ain't nuthin' happenin'." Ben Webster