Senta's immediate family appears in Book 0: Brechalon and Book 1: The Voyage of the Minotaur. Some of them make appearances later in the series.
Granny (Admeta Mae Goose): I didn't use Granny's name until book 5. I just liked that her name was Granny Goose. Granny loves kids and has cobbled together a family of her own grandchildren and the children of a couple of neices and nephews. All the kids in the household are related, but not necessarily siblings.
Bertice Haver is the oldest and is already working in Brechalon.
Geert McCoort is closest in age to Senta, and has to go to work in Book 0. His last name is a nod to author Frank McCourt, though I changed the spelling. His older brother is Maro McCoort.
Didrika Goose is a toddler who joins the family in Book 0 when her parents die. Her baby sister is Ernst Goose, and is still in a crib in Book 1.