How is Clear Light of Day a memory novel?

The term "memory novel" simply means that the narrative—both the overall arc and any related sub-plots—is heavily centered on the concept of memory. In particular, the novel's story heavily relies on the recollection of the characters. It is the literary equivalent of cinematic flashbacks used to present each character's backstory.
However, in this novel, the flashbacks aren't mere vignettes, but are the foundation of the overall narrative. Bim, the older sister of the protagonist, is stuck in the past, in which she recalls past injuries and perceived transgressions towards her. She also believes that she is still the power figure of the family. Their brother, Baba, is also stuck in the past, in which he is depicted as obsessing over his record collection from the 1940s. This symbolizes the fact that the siblings are yearning for a life they once lived and want to go back to that state of being.
Tara herself is not immune to nostalgia. She reminisces about their youth as a way to figure out the present-day tensions between them. In the novel, the purpose of recalling the past is not simply to leisurely stroll down memory lane, but is a way to analyze and make sense of the past, present, and the possibilities of the future.