When we pluralize words like son-in-law, mother-in-law, daughter-in-law, etc., we do that by simply taking the noun and adding an s.
So it’s mothers-in-law, fathers-in-law, sons-in-law, etc. For example, the author has two lovely daughters-in-law. (They are not daughter-in-laws.)
Some of you are screaming at me now and saying, “But law is a noun, too.” Aye, but there’s the rub: the phrase “in-law” is an adjective describing the kind of mother/father/son/daughter you’re talking about. So in that two-word construction, law is not a separate noun.
This week’s other funny/funky plural is a legal and/or governmental term: attorney general. In this phrase, general is an adjective, not a noun. It describes the kind of attorney we are talking about. (Insert tasteless lawyer joke here.] So we take the noun—attorney—and add an s when we want to make it a plural. So one attorney general, two attorneys general, three attorneys general (and that’s really all the attorneys anyone wants to deal with, yes?).
Don’t overcomplicate things here. Just find the noun and add the s there.