How to indicate possession by e.g. passers-by, mothers-in-law
"Mothers-in-law's" is a correct form. I am quoting the rule used in American grammar:
Also add an apostrophe followed by an s to the end of a singular compound noun that ends in any letter except s. For example:
- Queen of England's carriage
- King of Spain's castle
- Prime Minister of Canada's question
- mother-in-law's recipe
I suggest avoiding it. No matter how you put it, half of your audience will think you got it wrong and be distracted.
Try "the recipes of the mothers-in-law."