A Storm of Swords
In "A Storm of Swords", she plots to have Sansa taken to Highgarden to marry her grandson Willas. This plan is foiled by the Lannisters, who force Sansa to marry Tyrion Lannister. According to Littlefinger later in "A Storm of Swords", in order to shield Margaery from King Joffrey's cruelty, Olenna had actually been the one who murdered Joffrey at his wedding. Margaery would later go on to marry the younger brother, Tommen Baratheon.