I don't think movie got it wrong. Movie are based on real life experience and in real life most of the mother is law play the negative role. So, you can't say movie get it wrong. Maybe you were lucky.
But the good thing is that in the current time mother-in-law is quite good compared to old time as they try to treat better.
!PIZZA
Movies got it wrong on my end, and yes you can say that I am lucky likewise other persons. Not all mother in-laws are bad you know, but most times, movies only play the negative side