I always thought Flora and Desmond were some kind of relatively distant, not-so-rich relations. Flora married into the Collinses, but maybe her husband was some kind of cousin of Daniel's. Desmond is in the same generation as Daniel's children. I remember that when I was growing up (while the earth's crust was still warm), we were under strict orders to give every adult in the neighborhood the courtesy title of "Uncle" or "Aunt," even though they weren't relatives. So Desmond might be doing the same, just as Samantha addresses her father-in-law, Daniel, as Father Collins.
However, your spoiler raises all sorts of interesting questions. [spoiler]We know that the Collinses love to marry each other, so maybe Flora was born a Collins as well as marrying one.[/spoiler]