I am not familiar with any statistics that can tell that for sure, all I can say is that I have a daughter with DS and there was no history in my family. Is like the lottery!
don't know how comman but I am the mother of a DS and my hubbys nephew had a DS child that died at 2 yrs during heart surgery. So my hubbys brother was the grandfather of that child. I worked in a facility of adults with disabilities for many years, it was rare to have two children there that were related but not unheard of, with the genetic makeup it is possible that the faulty gene does exsist and gets passed on. I know of two families that had two each, but they were not DS but micro cephalic, I had genetic testing after my son was born and was told that he was a DS because of my age and that it was not a genetic thing, I did this so that my grandchildren could know and that their chances of having a DS baby would be no less or more than the average population.