Joff would need to punish Father, the lords would expect it, but perhaps he could send him back to Winterfell, or exile him to one of the Free Cities across the narrow sea. It would only have to be for a few years. By then she and Joffrey would be married. Once she was queen, she could persuade Joff to bring Father back and grant him a pardon.
Only... if Mother or Robb did anything treasonous, called the banners or refused to swear fealty or anything, it would all go wrong. Her Joffrey was good and kind, she knew it in her heart, but a king had to be stern with rebels. She had to make them understand, she had to!
I'll... I'll write the letters, Sansa told them.
With a smile as warm as the sunrise, Cersei Lannister leaned close and kissed her gently on the cheek. "I knew you would. Joffrey will be so proud when I tell him what courage and good sense you've shown here today."
In the end, she wrote four letters. To her mother, the Lady Catelyn Stark, and to her brothers at Winterfell, and to her aunt and her grandfather as well, Lady Lysa Arryn of the Eyrie, and Lord Hoster Tully of Riverrun. By the time she had done, her fingers were cramped and stiff and stained with ink. Varys had her father's seal. She warmed the pale white beeswax over a candle, poured it carefully, and watched as the eunuch stamped each letter with the direwolf of House Stark.