Basically what is the likelihood of an American fighting a close family member who's family had elected to say in Europe or for some other reason had not come to American
This isn't my area of specialty, but I don't really think you are going to find yourself a satisfactory answer.
There isn't really any way a historian could answer this question accurately. Asking about the chances of it happening suggests that you want to know the statistical likelihood, and that requires information that we simply do not have.