Sorry but I think I know. a bit of this...WC was never in line to the Dukedom...
WC was the elder son of a younger son who’s elder brother was very healthy....
Did I hear a loud “Molan Labe” from the audience? LOL.
Winston Churchill - Presumptive heir
http://www.vanityfair.com/style/2014/10/duke-of-marlborough-died-at-age-88
Btw, just to be clear , of course Winston Churchill was the presumptive heir UNTIL the elder brother had a child.