In NL this is not common either. This is not what keeps us safe. We use the mirror like any other country (but we do not forget). This topic has reached FP before. Original article and comments: https://news.ycombinator.com/item?id=12674533
We use the mirror like any other country (but we do not forget). Except that passengers in the back seats do not have mirrors. Although I never learned the Dutch Reach (but I don't have a driver's license), my dad always warned us as a kid that we should check for cyclists before opening the door.
I live in an area where the parking spots are on the left side of the road. I don't know how common it is for Dutch people to check the mirror for the passengers door. Maybe it just depends on your mother-in-law.