Camilla Salander is the estranged twin sister of Lisbeth Salander. She's hasn't been seen since their seventeenth birthday, and she and Salander don't get along. Apparently, she's very beautiful, and also something of a daddy's girl. Zala speaks of her with something like tenderness. Overall, she seems to be the complete opposite of her sister Lisbeth. Camilla and Lisbeth never had a good relationship, while Camilla thrived socially and academically, Lisbeth was the complete opposite. They began distancing from each other from then on.
They are both the daughters of Agneta Sofia Salander.
In The Girls in the Spider's Web, we finally find out what has become of Camilla, when she tries to seduce Blomkvist in order to gain access to her sister.