Who is Tullia from The Early History of Rome: Books I-V and what is their importance?

Tullia was one of Servius's daughters. Unable to inherit the throne directly, she fell into conspiracy with Tarquin, who began as her sister's husband. She and Tarquin murdered the other two, married each other and succeeded in their plot to attain the throne of Rome for themselves. They were not popular with the people, which led to a rather bloody reign for Romans themselves. Their son Sextus also turned out to be an able leader and a loyal son.