Only one of them will make it out alive…
Okay, Allie and Jill Gilchrist are the sisters Lauren's group rescues in Chapter 19. They're caught in the rubble of their home after the earthquake, and Lauren and company pull them out of the ruins, even though the two sisters are bloody and confused. Helping the two women makes Lauren and her friends targets, but they fight off the attackers who come, and Allie and Jill become members of the northward migrating Earthseed show.
Allie and Jill are 24-25 years old, poor, and they're running away from a life of prostitution. Yeah, their father was their pimp (19.100). That's gross, sad, and bad in all kinds of ways. Jill admits that the two of them are grateful that's Lauren's people saved them, but Jill and Allie are still suspicious at first of all this Earthseed business (19.106-118). They wonder if it's "Some kind of cult or something" (19.110).
Their suspicion is justified by their life backgrounds. Allie used to have a little boy named Adam (20.93-99), but her pimp father killed the infant. That's pretty horrifying. So it kind of makes sense later when Allie adopts Justin Rohr, the orphan he group finds (20.89-92). But back to their nasty father—Allie and Jill burned down their own home at one point, in the town of Glendale, aiming to escape him (20.99). When life is that bad, you get desperate.
Eventually, the two sisters begin to appreciate Lauren and Earthseed more. For instance, Allie starts asking real questions about the religion (21.115). It all seems to be going pretty well.
But then Jill gets killed during a battle between Lauren's group and some of the painted-faces freaks in Chapter 24: she's shot in the back while helping Tori (24.34-35). Minus one for the Earthseed crew. Not too long after that, Lauren comforts Allie (24.98-101), and things start looking up again. Once they reach Bankole's land, Allie's attitude toward Earthseed is a smiley one (25.18), and she—and Justin—agree to stay (25.39-40).
Allie and Jill fit into this novel in that they're good examples of the sort of people who need Lauren's help and a community to survive. The sisters have their own tragic stories, but once they meet up with Lauren, they get a family, so to speak, that wants to help them.