We never actually meet Miss Volker's sister (she doesn't even get a name), because she left her hometown recently, and now lives in Florida. She is an important presence in the book, though, since a key moment is when her former house is burned down. During her time in Norvelt, she was the head of the Federal Art Project, and taught art classes (ceramics and painting) (14.51). She also helped Miss Volker with the obituaries, like Jack now does.
We know from Miss Volker's stories that she's a kind and generous person who embodies the community ideals of Norvelt. Because she couldn't have her own child, she and her husband adopted a Japanese baby whose parents were being sent to an internment camp (14.52). Sadly, they only had the baby for six months before the government took him away. No one ever found out anything else about him, and Miss Volker's sister was heartbroken.