Sol and Ellie are like cool parents—the kind of parents who are fun when they're someone else's parents and maybe not so great when they're yours. And they're the perfect foils for Maddy and Az, David's parents:
In fact, this last part is the most important thing that Sol and Ellie do. When Tally is caught between her promise to Shay and her desire to be pretty, her parents tell her that she should help the secret police. (Note: It is almost never the correct option to help the secret police. If they're secret, it's probably because they're doing bad things.)
It's not so important that Sol and Ellie give bad advice—all parents give bad advice. What's important is that this is the first time that Tally recognizes that her parents aren't so smart: "For the first time in her life, Tally found herself listening to a middle pretty without being completely reassured, a realization that made her dizzy. And she couldn't shake the thought that Sol knew nothing about the outside world Shay had fled to" (14.49).
So Sol and Ellie help Tally change her mind by being clueless parents. Whew. Time for clueless parents everywhere to breathe a sigh of relief—they're still doing their job. Sort of.