Nope. It should be mothers-in-law. Although the whole word 'mother-in-law' itself is a noun, the changes in its plural should only be applied to the first word. This would include 'attorneys-general' and 'teachers-to-be'.
Since a lot have been learned, it is time for the exercise!
For the exercise, you can try this link. Irregular forms You will be able to know your score on the spot. Just ignore the misspelled word 'socre' (it should be score). ^^