From my Rex and Hannah Chronicles Wiki:Accidentally freed from Purgatory by someone fooling with a Ouija Board, the now remorseful spirit of Christy Jenkins returned to Earth in 2009. Christy was in search of her younger sister, Wilhelmina "Billie" Jenkins, whom she hoped to reconcile with. Christy’s search led her to Rex and Hannah, who she identified herself to. The name seemed familiar to Rex and he soon located references to it in some scrolls that he had captured during an Insurgent raid on the then Demon controlled Magic School, several years before.
The scrolls revealed that the reason that Christy had been kidnapped in the first place was because the Triad had a long range plan to overthrow the Source and rule the Underworld themselves. Since they would need the support of the Upper Level Demons to do so, the Triad figured that taking out the Charmed Ones was the way to achieve that goal. Hence their plan to use Christy and Billie against the Charmed Ones, a plan the Source knew nothing about, since it would have led to his downfall.
Seeking more information on Christy, Rex and Hannah consulted with Paige Matthews, who told them about the Ultimate Battle and how Christy had died, some three years earlier. In her years of Demon captivity, Christy had been slowly brainwashed by Dumain, a Demon working for the Triad, and was now converted to their cause. As a result of said brainwashing, Christy had slowly been turning Billie against the Charmed Ones. The Triad hoped that the combined powers of Christy and Billie would be enough to eliminate the Charmed Ones (and they hoped Christy and Billie would perish as well, leaving no loose ends). However, in the end, Billie realized the truth and would not go along with Christy any longer. After the Charmed Ones vanquished the Triad and Dumain, Billie tried to reason with Christy, but it was too late. Christy conjured a fireball and hurled it towards the Charmed Ones. Billie, on instinct, used her own powers to send the fireball back at Christy, incinerating her. Billie was devastated at the loss of the sister she had worked so hard to save, and, as a result, cut ties with the Charmed Ones and left San Francisco soon after.
Doing some leg work, Rex, Hannah, and Paige tracked Billie down to her new apartment in Seattle, Washington, and told her about Christy’s spirit.
At that point, Christy's spirit appeared, having followed Rex, Hannah, and Paige to Seattle. Although she was reluctant to listen at first, Billie soon heard Christy out and realized that Christy was indeed sorry for what had happened. Christy now realized that she'd been used and manipulated for years. Christy and Billie then finally reconciled and Christy, finally at peace, crossed over to the Spirit World.
Now at peace with herself, now that she had reconciled with Christy, Billie soon moved back to San Francisco and renewed her ties with the Halliwell sisters.