Thanks very much for your responses on this. This has allowed me to see two different ways forward on this. 1 - Elizabeth and Tim's method - Find descendants of Mary's husband Joe's siblings who are willing to test and see if they match Sam's son. If they do, the Sam's mother is one of the daughters of Joe and Mary. If they do not match, then it is evidence for Mary being Sam's mother (although we would really need to test several people and have them all not match in order to be sure it was not some fluke of atDNA inheritance). 2 - Belinda's method - See what the existing cousins' level of relationship is to Sam's son in terms of shared DNA, maybe identifying and testing a few other relatives who could shed more light on this. I am going to try method 2 first, since we have 7 relatives on Sam's mother's side who have already tested. So we might have enough to make a preliminary determination that way. But I ultimately see that method 1 is something we have to eventually do.     Wesley