My mom has a half-sister who she's never met. She's mentioned this offhandedly to me just once and this woman may be dead now for all we know.

I once asked about the large age gap between her parents. As her father was 15 years older than her mother. She told me that her dad was married once previously and had a daughter with his ex-wife. As I recall, my mom either wasn't told or didn't remember when my grandpa's previous marriage was or when his daughter was born. Being a devout Catholic, he didn't like to talk about his divorce and previous marriage much at all or have much contact with his ex-wife. For the most part he just quietly sent child support checks and was otherwise out of their lives as far as she knew.

The reason I figure that this half-sister may be dead is that my grandfather was born 109 years ago. (and died at 74) My maternal grandparents had kids late and so did my own parents (I'm 21, my mom is 58, and my dad would be 59 if he were alive) Anyway, figuring that my grandfather got married at 18 (though it's entirely possible given the era that he lived that he was even younger) and had a child within the first year or two of marriage as many couples did at the time, she could be around 90 years old.