Elizabeth and Jessica in a tug-of-war...

Chubby Robin Wilson has been following Jessica around for months. First she wanted to be her friend--now she wants to join Pi Beta Alpha, Sweet Valley High's snobby sorority.
When Elizabeth, Jessica's twin, nominates Robin for the sorority, Jessica is furious. Robin may be friendly and smart, but she's certainly not beautiful or popular enough to be a Pi Beta. Jessica's determined to find a way to keep Robin out.
But Elizabeth is just as determined to make Robin a sorority sister. Soon the twins are locked in a struggle that develops into the biggest power play at Sweet Valley High....

Jessica was stunned. "Robin Wilson, in Pi Beta Alpha? With us? With Lila Fowler? One look at that shape and they'll be calling us the Pi Butterball Alphas! Oh, really, Liz, are you completely absent from this universe?"
Elizabeth sighed. "Jessica, for heaven's sake, don't come unglued. I've promised, and that's that. Robin Wilson's going to be in the sorority."
"Oh, is she?"
"Look," Elizabeth said, "if I sponser Robin, and you're the president of Pi Beta Alpha - and her best friend - she's in."
Jessica's only response was to toss her hair and stomp out of the room. It' wasn't until the instant before she slammed the door to her room that Elizabeth though she heard a faint reply.
"We'll see about that."

Welcome to Sweet Valley High - a world of good girls looking for justice, bad girls conniving to rule, gorgeous boys trapped in the middle, and enough gossip to keep the school buzzing all year long

JESSICA WAKEFIELD is used to setting off sparks. But when she starts a new, highly selective club — the Sweet Valley High Beautification Committee — she begins a war with her twin sister.
Elizabeth knows that Jessica has more on her mind than simply cleaning up the SVH campus. It’s obvious that the Beauties are out to become the most elite group of girls at school, even if it means walking all over those they deem less worthy.
Only one Wakefield can reign supreme. And Liz is determined to show Jessica that the final say on beauty won’t come from a committee vote.

Jessica with a college guy? Elizabeth fears her twin has gotten in over her head this time. Is she right? All Night Long