We can consider the Lisbon girls as a unit because the narrator-chorus often lumps them together that way. If you've seen one Lisbon sister, you've seen them all. It isn't fair, but that's how the novel works—we don't make the rules, Shmoopers. It's part of the girls' tragedy—no one really sees them as real, distinct people. Just take a look at how the group of narrator boys introduces the protagonists:
The Lisbon girls were thirteen (Cecilia), and fourteen (Lux), and fifteen (Bonnie), and sixteen (Mary), and seventeen (Therese). They were short, round-buttocked in denim, with roundish cheeks that recalled that same dorsal softness. Whenever we got a glimpse, their faces looked indecently revealed, as though we were used to seeing women in veils. (1.9)
Early in the novel, the boys don't know much about the girls as individuals. They're just glimpsed through the window or at school, they hang out together, and they're just a mysterious, intriguing conglomerate of teenage femininity. They have a kind of mythic status, especially after the first sister's suicide—like the Kennedys, according to one boy's description.
None of us went to church, so we had a lot of time to watch them […] the five glittering daughters in their homemade dresses, all lace and ruffle, bursting with their fructifying flesh. (1.10)
They can't seem to tell them apart. Even Cecilia, in her diary, describes her sisters this way:
Instead, Cecilia writes of her sisters and herself as a single entity. It's often difficult to identify which sister she's talking about, and many sentences conjure in the reader's mind an image of a mythical creature with ten legs and five heads, lying in bed eating junk food, or suffering from visits from affectionate aunts. (2.13)
They hang together even more closely after their sister's suicide and take on a kind of mythic status to the boys.
Added to their loveliness was a new mysterious suffering, perfectly silent, visible in the blue puffiness beneath their eyes or the way they would sometimes stop in midstride, look down and shake their heads as though disagreeing with life. (3.4)
In one way, this is the story of all close siblings, who share the same family environment and spend tons of time together. But because the Lisbon girls are so isolated, they're even more dependent on each other or survival. The boys have a revelation the night of the party for Cecilia, the night of her suicide:
[…] for the first few seconds the Lisbon girls were only a patch of glare like a congregation of angels. Then, however, our eyes got used to the light and informed us of something we had never realized: the Lisbon girls were all different people. Instead of five replicas with the same blond hair and puffy cheeks we saw they were distinct beings, their personalities beginning to transform in their faces and reroute their expressions. (1.42)
What an insight. They're regular people?
The girls didn't appear as a group [after Cecilia's suicide] until Convocation. […] Mary, Bonnie, Lux, and Therese came to school as if nothing had happened. Once again, despite their closed ranks, we could see the new differences between them, and we felt that if we kept looking hard enough we might begin to understand what they were feeling and who they were. (3.23)
Still, they're constantly together after Cecilia's death, and people see them even more as a closed-off group, catching glimpses of the sisters at school but never really breaking through.
Various sightings of them at the time merged into a general image of their careful cluster moving down the central hallway. (3.75)
When Trip finally convinces Mrs. Lisbon to let the girls go to Homecoming on a group date (of course), the Narrators are surprised how normal the girls act once they're out of the house:
Even close up, the girls didn't look depressed. […] Who had known they talked so much, held so many opinions, jabbed at the world's sights with so many fingers? […] Somehow, too, they'd kept up on dating etiquette, through television or observation at school, so that they knew how to keep the conversation flowing or fill awkward silences. Their dating inexperience showed only in their pinned-up hair, which looked like stuffing coming out, or exposed wiring. (3.142-3.143)
Still, throughout the night, the boys experience the girls as a single group and see them hanging around together as "they."
The sisters can't live without each other—literally. Their last act as a group, the remaining four girls, is to kill themselves at the same time.
We knew that Cecilia had killed herself because she was a misfit, because the beyond called to her, and we knew that her sisters, once abandoned, felt her calling from that place, too. (5.40)
After their deaths, the sisters again became to many people just a jumbled group of girls who met a tragic end. The newspapers mix them up and make up stories about them that the boys know to be wrong. People searched for a common cause, or a single characteristic that explained the suicides—genes, neurotransmitters, family, PTSD. Their memories start to meld together even for the boys.
The indistinctness of the sisters mirrors the ambiguity of the narrators' "we" early in the novel. The boys refer to themselves as a group, too, all of them fascinated by the Lisbon girls and sharing the same kind of interests and experiences. Members of this "we" get more distinct as the novel progresses, but there's still this sense of a tight-knit group whose experience of watching and adoring the sisters bound them up in a permanent brotherhood.
A couple of the sisters remain relatively unknown to the readers throughout the novel; Eugenides doesn't spend a lot of time developing their characters. Lux and Cecilia get the most attention, but Shmoop is glad to share what we know, so we'll do some digging and see what we can come up with about each sister.
Keep in mind though, that the descriptions of each sister in the novel are pieced together from the observations of many different boys who saw the girls in different circumstances. So despite what we learn about each girl's personality and quirks, it's only a partial picture, synthesized from these collective observations.
Eugenides said in an interview that the girls were really "created by the intention of the observer, and there are so many points of view that they don't really exist as an exact entity" (source). He thought that, in the movie version, the girls should have been each played by a number of different characters at different times, depending on who was talking about them. That's the impression he was aiming for—he called them "shape-shifters."