@Paul Raven I was thinking of exactly the same examples when I saw the title of the thread. The Frames became a core family and were represented through Frankie and Sharlene until the end of the show. Clarice is such an interesting case because she came on the scene as a supporting character, but wound up having not only her family (the Hobsons) but also her husband's family (the Ewings) built around her. It could be argued that at one period in the late 1970's to the early 1980's from her involvement in plots with Ada, Iris, Cecile, and Sally, she was the central hub of the show.
Also, I appreciate the distinction you made between characters who were introduced as the first member of an already planned family, versus those whose family were created as after thoughts. The after thought families usually had more complex family trees to explain all of the long lost siblings, wives, and children who just happened to move to the same small town (e.g. Michael Hudson's clan).