Clue #4 and A Bit of Family Trivia

"Mark & Nancy would like to meet you there."
-Mom

Mark and Nancy are my uncle and aunt on my mom's side.

When I was ten, my folks sent me back east to visit with relatives for two weeks (my whole extended family is pretty much in Ohio). One of those weeks was spent with my aunt Nancy, who is my godmother. During that time I called Mark "uncle Mark" and, in an impulsively wise ass moment, he explained to me that he was not, in fact, my "uncle" because we had no blood relation. Instead, he was just my aunt's husband.

Ever since, any birthday cards, etc. are always addressed to/signed by my uncle(usually underlined at least three times) Mark and aunt Nancy.