I had trouble finding more information on Lady Sun since she was one of many wives of Liu Bei. The one most commonly discussed is Lady Gan. One source writes, "In 196, when he entered Jingzhou (in the region of the present-day provinces of Hubei, Hunan, and southwestern Henan Provinces), he had at least one principal wife, Lady Mi, a sister of his supporter Mi Zhu. Lady Sun, a younger sister of Sun Quan (182– 252; r. 222– 252), later to become emperor of the kingdom of Wu, also became a principal wife of Liu Bei, possibly in 209 (vide Wu, Empress of the Former Sovereign of Shu).

It later continues:

"The warrior hero Liu Bei, who claimed kinship to the Han royal house, had gained control of Yizhou by the early years of the third century. He had also joined forced with Sun Quan (182– 252; r. 222– 252) of the southeastern state of Wu and in 209 had married Sun Quan’s younger sister, Lady Sun. Lady Sun is described as a formidable young woman, as courageous as her four famous brothers and with over 100 female attendants capable of bearing arms. Tradition has it that every time Liu Bei, then almost fifty, laid eyes on her his heart turned cold, even though she was barely in her twenties. Believing her capable of starting an uprising in support of her brother, Liu Bei’s chief strategist advised him to return her to her home state of Wu. Then, when Sun Quan learned that Liu Bei was preparing a western campaign in 211, he sent a boat to bring his sister back to Wu. Before she left in 212, Lady Sun attempted, unsuccessfully, to kidnap the heir apparent, Liu Shan (known to history as the Later Sovereign [houzhu] of Shu, 207– 271; r. 223– 263), over whom she had exercised guardianship."

Over the leaving:

"The warrior hero Liu Bei, who claimed kinship to the Han royal house, had gained control of Yizhou by the early years of the third century. He had also joined forced with Sun Quan (182– 252; r. 222– 252) of the southeastern state of Wu and in 209 had married Sun Quan’s younger sister, Lady Sun. Lady Sun is described as a formidable young woman, as courageous as her four famous brothers and with over 100 female attendants capable of bearing arms. Tradition has it that every time Liu Bei, then almost fifty, laid eyes on her his heart turned cold, even though she was barely in her twenties. Believing her capable of starting an uprising in support of her brother, Liu Bei’s chief strategist advised him to return her to her home state of Wu. Then, when Sun Quan learned that Liu Bei was preparing a western campaign in 211, he sent a boat to bring his sister back to Wu. Before she left in 212, Lady Sun attempted, unsuccessfully, to kidnap the heir apparent, Liu Shan (known to history as the Later Sovereign [houzhu] of Shu, 207– 271; r. 223– 263), over whom she had exercised guardianship."

There is another segment related to his death that could provide insight: "Liu Bei knew that the people would slow down the retreating forces, especially since they had to cross a river to escape. Still, he could not bear to abandon them. In the end, while fending off Cao Cao's troops as he tried to get all of his people safely across the river, Liu Bei lost most of his army and his wives.

Liu Bei later regrouped his forces and defeated Cao Cao by forming an alliance with Sun Quan of the Eastern Wu in the battle of Red Cliff. Zhuge Liang, the master strategist, played a pivotal role in this successful campaign."

Thus, I would assume Lady Sun went to Wu after Liu Bei's death. But I'm running out of time to confirm this with documentation. Hopefully the sources below provide additional information.

Sources: Biographical Dictionary of Chinese Women: Antiquity Through Sui, 1600 B. C. E. --618 C. E

Wu, D. (2013, Sep 23). Liu bei: Humane and compassionate king. The Epoch Times

Edit: I Have to run to a meeting, I just realized I copied the citation instead of a quote from the source. I'll edit later.

Edit 2: Found more information.