During his military campaign against the Tatars, Genghis Khan fell in love with Yesugen and took her in as a wife.
She was, along with Yesui, the daughter of Yeke Cheren, a Tatar leader executed by the Mongol forces.
[1] “Being loved by him, Yisügen Qatun said, ‘If it pleases the Qa’an, he will take care of me, regarding me as a human being and a person worth keeping.
But my elder sister, who is called Yisüi, is superior to me: she is indeed fit for a ruler.’ ”[2] -The Secret History of The MongolsDespite her willingness to be married to Genghis Khan, Yesugen declared that her older sister, Yesui, was “superior” to her.
Genghis Khan searched for and found Yesui, and Yesugen yielded her position to her sister.