Casey was adopted by Sam from China. Sam may have adopted her sometime after filing for divorce from Ginny seven years prior. It is implied that Marilyn, Sam's mother who is firmly traditional is having difficulty treating Casey as a "real" granddaughter, that she often makes remarks about her Asian roots. Sam mentions Marilyn bowing to Casey at Thanksgiving.
In A Little Christmas Queer, Casey attends a Christmas dinner at her grandparent's house in Connecticut.