In the book "Twilight" by Stephanie Meyer, the Cullen family and the Swan family are structured very differently from one another. When looking beyond the extreme abnormalities of the Cullen family, like the kids pairing up in couples, their ages, or the fact that they're all vampires, the Cullen's are actually portrayed as a very ideal, "All American" family. Contrasting from the Cullen's, Bella's family is depicted as broken and unhappy. There's definitely irony in this, which I think Meyer did on purpose. She wanted to highlight the fact that although the Cullen's are unorthodox in many ways, they are still near perfect as a family unit.
I think one reason Meyer portrays the Cullen family in such an ideal way is to prove that a family doesn't need to share the same blood and genetics for them to be happy and successful. None of the Cullen's are related, however, they appear to be living the American Dream. Carlisle works as a doctor and acts as the father figure of the family. Like any "All- American" dad, Carlisle provides financial stability for the family while taking on the role as protector. Likewise, his wife Esme acts as the mother figure, taking on the role as the dutiful, loving wife who stays home and tends to household matters. As the parental figures, Carlisle and Esme support their "kids" at all costs and want nothing more than for them to be happy. This is shown in chapter 15 when Bella expresses her concern for meeting Edward's family. Edward says, "Carlisle and Esme are happy to see me happy. Actually, Esme wouldn't care if you had a third eye and webbed feet." .
The Cullen's are a considerably wealthy, attractive, and highly educated family who are entirely devoted to one another. It sounds almost too good to be true! Bella even comments at one point saying, "It seemed excessive for them to have both looks and money." Bella, who is blood-related to both her mother and father, comes from a broken family filled with divorce and distant relationships (both physical and emotional).