Marjorie did her work very coldly and succinctly in three sentences.
"You may as well get Warren out of your head," she said coldly.
"What?" Bernice was utterly astounded.
"You may as well stop making a fool of yourself over Warren McIntyre. He doesn't care a snap of his fingers about you."
For a tense moment they regarded each other – Marjorie scornful, aloof; Bernice astounded, half-angry, half-afraid. Then two cars drove up in front of the house and there was a riotous honking. Both of them gasped faintly, turned, and side by side hurried out.
All through the bridge party Bernice strove in vain to master a rising uneasiness. She had offended Marjorie, the sphinx of sphinxes. With the most wholesome and innocent intentions in the world she had stolen Marjorie's property. She felt suddenly and horribly guilty. (92-93)
This horribly awkward scene demonstrates the fact that betrayal, unfortunately, can be either intentional or unintentional. Marjorie's flip towards Bernice is precipitated by a betrayal Bernice didn't even intend to perpetrate – her "theft" of Warren.
"When you going back to kindergarten, Otis?" some one had asked.
"Me? Day Bernice gets her hair bobbed."
"Then your education's over," said Marjorie quickly. "That's only a bluff of hers. I should think you'd have realized."
"That a fact?" demanded Otis, giving Bernice a reproachful glance.
Bernice's ears burned as she tried to think up an effectual come-back. In the face of this direct attack her imagination was paralyzed.
"There's a lot of bluffs in the world," continued Marjorie quite pleasantly. "I should think you'd be young enough to know that, Otis."
"Well," said Otis, "maybe so. But gee! With a line like Bernice's –"
"Really?" yawned Marjorie. "What's her latest bon mot?"
No one seemed to know. In fact, Bernice, having trifled with her muse's beau, had said nothing memorable of late. (94-96)
Oooh, ouch. Marjorie's vengeance is horrifyingly efficient – having decided to turn against her cousin, she takes Bernice down in a devastatingly systematic manner. Bernice doesn't even know how to process this betrayal; there's no way she can recover enough to out-Marjorie Marjorie this time.
Out of the group came Marjorie's voice, very clear and contemptuous.
"Don't worry – she'll back out!"
"Come on, Bernice!" cried Otis, starting toward the door.
Four eyes – Warren's and Marjorie's – stared at her, challenged her, defied her. For another second she wavered wildly.
"All right," she said swiftly, "I don't care if I do." (102-104)
Bernice finally manages to process Marjorie's back-stabbing attack and stand up for herself – what started as her cousin's betrayal now erupts into war.
Marjorie turned swiftly and with serpentlike intensity to Warren.
"Would you mind running me down to the cleaners?" she asked. "I've simply got to get a dress there before supper. Roberta's driving right home and she can take the others."
Warren stared abstractedly at some infinite speck out the window. Then for an instant his eyes rested coldly on Bernice before they turned to Marjorie.
"Be glad to," he said slowly. (115-116)
Marjorie's deceptive nature emerges here in the adjective "serpentlike," as we see the final act of her betrayal unfold – she reclaims Warren.