Step mother & Mother-in-law

Here is a question of Dave related to family bonds and how to name them in French:

What are step parents in French? My text says that 'step mother' and 'mother in law' are both 'la belle mère' but it seems like that could be rather confusing..

Indeed Dave, I can understand the confusion here...
I confirm you that both "mother in law" and "step mother" mean belle-mère in French

Your belle-mère can be your father's new wife, or can be the mother of your husband/wife. In French we use the same word to express the both ideas.
That's the same regarding fathers -> beau-père means at the same time "step father" and "father in law"

Step parents are beaux-parents in French

But this simple rule isn't the same for all family members!
As in english, we have in French several ways to name our "step broters/sisters,..."

A step brother = Un demi-frère (is litterally a half-brother) 
He's generally the son of your step mother/father
A brother in law = Un beau-frère
He's generally the husband of your sister, or your spouse/husband's brother

Moreover, to add some grammar details, be careful to the plural form of these words. As you maybe know you have in French plenty of exceptions...

demi-frère -> demi-frères (the word "demi" keeps its singular form)
demi-soeur -> demi-soeurs

BUT pay attention to:

belle-mère -> belles-mères (the word "belle" has to be declined to plural)
beau-père -> beaux-pères

Don't hesitate to comment for all additionnal information you may need :)

Hey, to add a small slang touch, do you know how to call a step mother or a mother in law in slang?
= une belle-doche ! or belledoche
And what about the brother in law?
= un beauf ! (short word for beau-frère)