I have a similar problem where one of my older family members refuses to believe that one ancestor was NOT an only child. He had an older brother who seemed kind of wayward--illegitimate child, then married (maybe? haven't found a marriage record...) the mother, had a slew of kids with her, divorced her, and then married (again, maybe... no record) someone else while all his minor kids from the first marriage lived in orphanages all over the country. It doesn't matter that I have a birth record (showing birth parents), baptismal record, census records (showing him in the same household as said brother and parents), and his death record, showing parents' names (with mother's maiden name, even), and that I've spoken to a descendant of said person. No, she's pretty sure that so-and-so was an only child, and his father (the ancestor who only had a brother on paper, apparently) was, too. From what the descendant I've spoken to has said, the family was very strained and isolated from the rest of the family, so it's not like they would have been at all of the family gatherings. She still hasn't been able to track down all of said person's children (born 1910-30). Luckily, the relative in question is not on WikiTree, so I have the "non-existent" uncle in here and don't talk genealogy with her anymore (there have been a few other cases with her of "That doesn't match what I remember being told over a decade ago...").