Why does Cersei sleep with her family?

Why does Cersei sleep with her family? She informed Jaime that their father planned to marry him to Lysa Tully and persuaded him to join the Kingsguard in order to be near her, after the twins had slept together for the first time. Thus started an affair that continued throughout Cersei’s marriage to Robert Baratheon.

She informed Jaime that their father planned to marry him to Lysa Tully and persuaded him to join the Kingsguard in order to be near her, after the twins had slept together for the first time. Thus started an affair that continued throughout Cersei’s marriage to Robert Baratheon.