Who is Uncle Viktor, Aunt Nadezhda from The Madonnas of Leningrad: A Novel and what is their importance?

The rigidly intellectual Viktor and the emotionally volatile Nadezhda take the young Marina into their home after her parents are arrested (and presumably executed) as political dissidents. She takes them with her when she and the other staff of the Hermitage Museum move into its basement bomb shelters and takes care of both their bodies when they each, separately, die as the result of the hardships they experience there. The actions of all three characters can be seen as another manifestation of the book's thematic interest in the power of loyalty.