I haven’t (so far) found anything in the article about the twins rolling their tongues. We were taught in Physiology and Anatomy class that this is a genetic trait. I have Celtic heritage and I cannot roll my tongue. It infuriates me that I can’t roll my tongue. I believe that NOT being able to roll the tonge is a recessive trait.
As I remember from school a pair of truly identical twins would have all the same recessive and dominant traits. HOWEVER, twins can LOOK identical and still be fraternal. They do have the same parents and some siblings have a remarkable resemblance without being identical twins.