How does the relationship among Agatha, Felix and their father compare with the relationship between Victor, Elizabeth, and Victor's father?
Both the DeLacey and the Frankenstein families are close, loving families, although Felix and Agatha are much more devoted to their father and each other than is Victor Frankenstein to his family.
Initially, in Chapter 1 of Shelley's novel, narration about the Frankenstein family describes them as in relationships that involve the one cared for and the caretaker. For instance, Alphonse Frankenstein comes "like a protecting spirit" to the rescue of Caroline Beaufort who has been orphaned by the death of her father, a true friend of the senior Frankenstein. Further, Caroline Frankenstein rescues Elizabeth Lavenza, saving her from a life of poverty, and she is received by Victor as though she is a cousin. There is love in this family; as Victor tells Walton, "Everyone loved Elizabeth."
Similarly, the DeLacey family, although having fallen upon misfortune, are a loving, caring family. The children of M. Delacey, a man who has lost his affluent social position in France and is now blind, are extremely respectful and devoted to him, and care for him as best they can; for instance, they go without food so that their father can eat, and they do what they can to cheer him and protect him. For example, when the creature enters the cottage in their absence, hoping to have human contact by speaking to M. DeLacey, they attack the creature upon their return and chase him out of the cottage, fearing that he means harm to their father.
Whereas the DeLaceys seem of all one mind, there is, however, diversity in the thinking of Victor, Elizabeth, and Alphonse Frankenstein. For one thing, a young Victor becomes greatly taken with the writings of Cornelius Agrippa, but his father tells him not to waste his time reading these writings: "...it is sad trash." Later, he rejects Agrippa, but he involves himself in yet another aspect of science of which his father would not approve as he commences upon the creation of a living being.
Of course, unlike Agatha and Felix DeLacey, Victor Frankenstein possesses a selfish pride that keeps him from preventing the death of his brother William and his cousin Elizabeth. Nor does he admit to his responsibility for these tragedies. He also does nothing to prevent Justine's condemnation, and he later places Elizabeth's life in danger by marrying her. Believing that the creature intends to kill him for having destroyed the female creature no matter what he does, Victor decides to have what little joy he can before his inevitable death. "Well, be it so," he says, and he marries Elizabeth. Unfortunately, this selfish attempt to attain some happiness results in not his death, but that of Elizabeth.
Certainly, then, the simple, loving relationships in the DeLacey family differ from the loving relationships of the Frankenstein family that become tragic because of the selfish interests of Victor.