I Took the Main Character's Sister at the Start, Now the Heroine is Full of Regret?

Ning Bei is a cultivator at the Heavenly Martial Academy on the Cangxuan Continent, trapped in a cycle of abuse by his fiancée, Lin Yueyao. In his previous life, he devoted all his resources to her, only to be betrayed when she favored Qin Yu, a part-time disciple and the world's designated Child of Destiny. Lin Yueyao eventually divorced Ning Bei publicly, causing his family's ruin and his own suicide. Upon reincarnating to the moment of their conflict, Ning Bei rejects his submissive role. A Revenge System activates, binding to him to help him defy fate and destroy his nemesis, Qin Yu.

The system reveals that Qin Yu's rise depends on the intervention of his adoptive sister, Qin Jiujiu, a reincarnated Great Emperor and the academy's top prodigy. Qin Yu plans to save her from a qi deviation, which will earn her loyalty and lead her to remodel his meridians. Ning Bei intercepts this opportunity by using a special pill provided by the system to induce a similar crisis in Qin Jiujiu. He enters her quarters under the guise of concern and, taking advantage of her altered state, consummates a relationship with her. This act severs the karmic link between the siblings and establishes a secret bond between Ning Bei and the Great Emperor reincarnate.

Ning Bei uses the system to enhance his own cultivation, tempering his meridians and advancing to the mid-Foundation Establishment stage. When Qin Yu attempts to claim kinship with Qin Jiujiu to gain status, Ning Bei publicly humiliates him, ordering disciples to beat the part-time disciple. Lin Yueyao, blinded by her devotion to Qin Yu, demands Ning Bei apologize. Instead, he accepts her threat to annul their engagement, publicly dissolving the relationship to the relief of the academy. Qin Yu and Lin Yueyao misinterpret his actions as jealousy, remaining convinced he will eventually beg for forgiveness.

Qin Jiujiu, grateful for Ning Bei's help and feeling guilty over the loss of her purity, gifts him a Heaven-rank cultivation technique, the Divine Sun Art. Their relationship deepens as they navigate the academy's politics together. During the selection for chief disciple, the Divine Mirror trial unexpectedly activates at Inferno difficulty, a level not seen in over a century. While other disciples perish or flee, Ning Bei and Qin Jiujiu cooperate seamlessly. Ning Bei uses a system item to pluck the Sacred Fruit, saving the remaining participants.

After obtaining the fruit, Ning Bei is pulled into a secret realm where he encounters the remnant soul of the Divine Mirror's creator. The soul intends to sacrifice them to restore its power. Qin Jiujiu remains by his side, and together they dismantle the sacrificial array. Ning Bei uses a Soul-Melting Talisman to absorb the remnant soul's energy, preventing it from taking over their bodies. They refine the absorbed energy together, both breaking through to the late Foundation Establishment stage. They agree to present their survival as a joint inheritance to the academy authorities.

Upon emerging, the elders are astonished by their survival and breakthrough. Ning Bei offers the title of chief disciple to Qin Jiujiu, citing her superior talent, which earns him widespread praise for his magnanimity. However, Qin Jiujiu insists Ning Bei receive the title, arguing that his actions were decisive and brave. The academy is left to decide how to handle the dual legacy. Meanwhile, Qin Yu and Lin Yueyao continue to plot against Ning Bei, unaware that their target has become a major power within the academy, closely allied with its most formidable genius.

The story follows Ning Bei's systematic dismantling of the original plot's advantages. By securing Qin Jiujiu's loyalty and wealth, he accumulates resources and status that Qin Yu cannot match. The Revenge System continues to reward him for altering fate, unlocking a shop with powerful items. As Ning Bei rises, the gap between him and the Child of Destiny widens. Qin Yu's attempts to undermine him are consistently thwarted by Ning Bei's foresight and the support of his new ally. The narrative shifts from a tale of betrayal to one of calculated revenge and strategic ascent, with Ning Bei positioning himself as the true protagonist of his own destiny.