These characters are on the maternal side of the Finzi-Contini family as the main story-line of the novel takes place. The parents of the two young Finzi-Continis, Micol and Alberto, are Ermanno Finzi-Contini and his wife Signora Olga (née Herrera). The Herrera family consists of Signora Olga, her mother Signora Regina, and Olga's two "bachelor brothers," Giulio, a doctor, and Frederico, an engineer. The brothers are "from Venice" but often visit the Finzi-Contini estate in Ferrara, where virtually the entire action of the novel takes place.
All four of the characters named in your question are actually relatively minor ones who have only a background presence. The main story-line involves the narrator's unrequited love for Micol Finzi-Contini. They meet as children, since their families already know each other as members of the close-knit Jewish community in Ferrara during the 1920s and 1930s. When racially-discriminatory laws are passed against Italian Jews in the late 1930s, the narrator and other young Jewish people who have been excluded from belonging to various organizations (such as athletic clubs) begin meeting at the Finzi-Contini house, where the huge garden has a tennis court. The narrator, who is Jewish but from a family of modest means, becomes close to Alberto, Micol, and a Gentile friend named Malnate, who works as a chemist and is an ardent socialist and anti-Fascist. Though the narrator imagines his association with Micol can become more than a mere friendship, when he begins to make advances she basically tells him it's never going to happen. Later, he realizes Micol is in a relationship with his close friend Malnate.
As stated, Olga (though she is the mother of Micol and Alberto), Regina (the grandmother), and the uncles (Olga's brothers) Giulio and Frederico, do not really have prominent roles. Late in the story Alberto becomes ill, and though Uncle Giulio, the doctor, can at first find nothing definitely wrong with him, Alberto eventually dies of a condition called lymphogranuloma. He thus becomes the only member of the present Finzi-Continis to be buried in the family tomb in Ferrara, alongside his brother Guido who had died of polio as a small child 25 years earlier. The rest of the family are all eventually deported in 1943 to a concentration camp and killed.