At the beginning of the novel, Tris's family seem like perfect members of the Abnegation faction. But soon enough, that all falls to pieces and their family unit is ripped to shreds.
Hmm. Maybe they're not perfect Abnegations after all. But if they're not here to represent the virtue of selflessness, what are they doing here?
Sure, everyone in the Prior family is better at Abnegation than Tris. Mama Natalie works with the poor factionless people (4.30). Papa Andrew helps run the government, including trying to help the factionless (11.36). And brother Caleb is just naturally selfless, which is he shows when he does things like give up his bus seat for other people (1.17). When Tris thinks about her brother, she thinks, "What irritates me most about him is his natural goodness, his inborn selflessness" (4.25).
How freaking annoying is that? We're being totally serious here. From the very beginning of the story, we hear how great the Prior family is and how terrible Tris is at being Abnegation. Mom "is well-practiced in the art of losing herself. I can't say the same of myself" (1.3). So we're not doing rocket science-level literary analysis when we say that the family is here largely to make Tris feel guilty—we're just reading the words where she says (over and over) "I feel guilty that I'm not as good as my family at this."
On top of all that, she also notes that her family is better looking than her. She tells us that her mom is pretty (1.13), her brother would get all the girls if he weren't Abnegation (1.16), and that Caleb looks like dad, so dad was probably quite the looker in his day (1.16). By contrast, Tris thinks she's not pretty (8.89) So really, the rest of the Prior family makes Tris look bad both because they're prettier than she is and they're better Abnegation than she is.
So when we first meet them all, the Prior family members seem like foils to Tris: they're awesome and pretty and selfless, which shows off how she is none of these things.
Ah, if only it were that simple. Tris's family, in the end, does way more than just highlight her flaws. After all, much of the book consists of Tris coming to understand just how complex she and other people are, and that includes her family.
For example, her brother Caleb, the perfect son and selfless Abnegation? He jumps faction and joins the hated Erudite. And it's not like Caleb joins the Erudite because he thinks it'll be best for mom and dad—he does it because he's taking care of number one: "We should think of our family. […] But we must also think of ourselves" (4.64). Caleb isn't the perfect Abnegation son. He lies and has secrets, just like Tris.
And dad? He's got some secrets, too. When Tris asks her dad if he's ever taken a bullet out before (primo father-daughter bonding time), he responds, "The things I know how to do might surprise you" (36.32). Well that's… alarming. Dad may be pretty good at the whole Abnegation thing—"being selfless, sacrificing himself, taking the stairs so that others can take the elevator"—but even he has a secret past that might shock his kiddos.
But mom totally takes the cake when it comes to keeping secrets. She may seem like the perfect Abnegation, but she has a whole other side that doesn't fit in with that. For one thing, the whole "faction before family" idea? Natalie Prior is not having any of that mess. She says over and over that she loves her "family" no matter what choices they made, like when she tells Tris, "You're my daughter. I don't care about the factions" (35.32). Not exactly what you would expect from someone whose entire identity is faction-based.
Plus, while Natalie might seem like a perfect Abnegation mom to Tris, she was born Dauntless. And she still knows how to fire a gun and save her daughter from drowning (15.115, 35.18). They don't teach that in Abnegation school, but they do offer an elective on that in Dauntless. In fact, just like Tris, Natalie Prior is Divergent (35.37). So Tris spent all this time beating herself up for not being like her mom—when, in fact, she's exactly like her mom, in more ways than she ever imagined.
So the Priors start out seeming like perfect Abnegations. But the more we learn about them, the more we see that they don't fit easily into the pigeonholes we've cleared out for them. They're not foils that show us how bad Tris is; they're more like mirrors that reflect back some of the same problems and issues that she has.
But here's our final question: both mom and dad give their lives to save Tris and help her succeed in shutting down the Erudite-Abnegation massacre. Did they do that because they're selfless Abnegation? Or because they're loving family? Or is it a little bit of both?