15She Betrayed Her Family
When Ned Stark began to the uncover the truth behind Cersei and Jaime Lannister's incestuous relationship, he realized that his family might be in danger. He prepared a secret escape from King's Landing for his family and servants.
Ned had a ship ready to take them out of the city at a
moment's notice. When Sansa became aware of this plan, she told Cersei, as she wanted to remain in King's Landing and marry Joffrey.
Sansa hoped that Cersei could mend whatever issues she was having with Ned and keep the engagement on. This tipped off Cersei that Ned was planning something, which caused her to accelerate her own plans. Sansa had inadvertently betrayed her own family, which led to the death of her father.
This story was removed in the show, due to the fact that all of the characters had their ages increased by a few years in the adaptation. An older Sansa wouldn't have been gullible enough to betray her family in such a way, so the scene was left out.