The wife of the Chinese dissident Liu Xiaobo is preparing a legal challenge against authorities after her husband and brother were jailed for 11 years and she was made a prisoner in her home.

Lawyers representing Liu Xia said she would sue Beijing police for illegal house arrest and prepare a petition for her husband's subversion case to be retried.

Her brother, Liu Hui, has filed an appeal against the 11-year prison sentence handed down to him this month for fraud. He was jailed over a real-estate dispute, and the family's supporters say his sentence is further punishment of the Nobel peace prizewinner's family and an attempt to intimidate fellow activists.

Shang Baojun, one of Liu Xia's lawyers, said her decision to pursue legal action was driven by her frustration over the way authorities had treated her family, and that her brother's conviction in particular had led her to speak out.

"Her biggest worry at the moment is her brother. She feels guilty for causing such huge trouble and negative influence to his family," he said. "She says would do anything to alleviate her brother's suffering."

The move marks the latest step by Liu Xia to challenge the alleged persecution of her family. A week ago, her lawyers released an open letter she wrote to the Chinese premier, Xi Jinping, to protest against her brother's sentence.

Since her house arrest Liu Xia has had few opportunities to express herself amid fears of further retaliation. She was permitted a rare opportunity to leave her flat on Thursday to speak to her lawyers, handing them a letter authorising them to prepare a legal petition for her husband's case be reopened, Shang said.

She also gave her legal team a separate letter entrusting them to represent her in suing the Beijing police for keeping her under house arrest since her husband won the Nobel prize in 2010.

Liu Xiaobo was jailed on subversion charges brought after he called for sweeping changes to Beijing's one-party communist political system. He was convicted in 2009 and a court dismissed his appeal in early 2010.

In 2010, the Nobel committee awarded the peace prize to Liu Xiaobo, infuriating the Chinese government, which denounced the accolade, called him a criminal and locked up his wife in her home.