Information:  - Emperor Hui of Han (210188 BC) was the second emperor of the Han Dynasty in China. He was the second son of the first Han emperor, Han Gaozu and Empress Dowager Lü. He is generally remembered as a weak character dominated by his mother, Empress Dowager Lü, personally kind and generous but unable to escape the impact of her viciousness. He tried to protect Ruyi, Prince Yin of Zhao, his younger half-brother, from being murdered by Empress Dowager Lü, but failed. After that he indulged himself in drinking and women and died at a relatively young age. Empress Dowager Lü installed two of his sons, Liu Gong and Liu Hong (known collectively as Emperors Shao of Han), the sons of the Emperor's concubine(s) after he died without a designated heir. Emperor Hui's wife was Empress Zhang Yan, a niece of his by his sister Princess Yuan of Lu; their marriage was the result of insistence by Empress Dowager Lü and was a childless one.  - Lü Zhi (241180 BC), courtesy name Exu, commonly known as Empress Lü and Empress Dowager Lü, or formally Empress Gao of Han, was the empress consort of Emperor Gaozu, the founder and first ruler of the Han Dynasty. They had two known children, Liu Ying (later Emperor Hui of Han) and Princess Yuan of Lu. Lü Zhi was the first woman to assume the title Empress of China. After Emperor Gaozu's death, she was honoured as Empress Dowager and later as Grand Empress Dowager () during the short reigns of Emperor Hui and his successors Liu Gong (Emperor Qianshao) and Liu Hong (Emperor Houshao).  - Liu Ruyi ( simplified Chinese :  ; traditional Chinese :  ; pinyin : Li Rúyì , 208 BC -- 194 BC ) , also known as Prince Yin of Zhao (  , created 198 BC ) was the third son of Emperor Gao of Han and only son of Concubine Qi . In 200 BC , he was made Prince of Dai after Liu Xi was demoted for fleeing his Dai province from Xiongnu forces . In 198 BC , Ruyi replaced Zhang Ao as Prince of Zhao after Ao was falsely accused of conspiring rebellion . After Emperor Gao of Han fallen ill early in 195 BC , he discovered Empress Lü Zhi 's plot to kill Ruyi , and appointed Zhou Chang as prime minister for Zhao province to protect him from Lü . Emperor Gao died later the year and Emperor Hui of Han continued Prince of Zhao 's protection by halting multiple attempts from Empress Dowager Lü on Ruyi 's life . When Emperor Hui left for a winter hunting trip in 194 BC , Empress Dowager Lü had Ruyi killed .  - Zhang Yan (died 163 BC), known formally as Empress Xiaohui () was an empress during the Han Dynasty. She was the daughter of Princess Yuan of Lu (the only daughter of Emperor Gao (Liu Bang) and his wife Empress Lü) and her husband Zhang Ao (, son of Zhang Er), the Prince of Zhao and later Marquess of Xuanping.    What is the relationship between 'liu ruyi' and 'princess yuan of lu'?
The answer to this question is:
sister