Is handwriting style heritable? My daughter (11), wife, and mother in-law all have extremely similar handwriting.

Why is this? Is handwriting a inheritable trait?
Your discussing the cog sci called graphology. It doesn't really work in general cases but its a nice idea. I would say its a happy accident that they look alike. My letter R look like my father's R because I wanted to please him at that age so I mimicked his handwriting.

Graphology is the study of handwriting. As a theory or practice for inferring a person's character, disposition, and attitudes from their handwriting, graphology is generally considered pseudoscience.
Graphology has been controversial for more than a century. Although supporters point to the anecdotal evidence of thousands of positive testimonials as a reason to use it for personality evaluation, most empirical studies fail to show the validity claimed by its supporters.

-wikipedia