“We believed we needed to save the rest of the world from the hell of capitalism,” she said.

Ms. Li, also a “princeling” child of a party official, said that although there has been a long history of enmity between her and Ms. Gu, her son never conspired to murder Ms. Gu.

Another family member confirmed that Ms. Li’s brother had met with Mr. Bo and had been told of the alleged plot. He also insisted the son was innocent. The son and his uncle both declined to comment. Mr. Bo and Ms. Gu are under detention.

Although she has no proof, Ms. Li said she suspected Ms. Gu was the one who first blamed her son for the perceived murder plot, and the so-called forensic evidence might have been provided by Wang Lijun, the former police chief convicted of helping cover up Mr. Heywood’s murder. Ms. Li said she feared Ms. Gu wanted to have her first son arrested or harmed.

“She can be that paranoid,” Ms. Li said. As for Mr. Bo, she said, he was “good in nature and didn’t want to believe this evidence.”

Ms. Li spoke with nostalgia of her romance with Mr. Bo, which began when the two met in 1975, at the end of the Cultural Revolution. Ms. Li said she did not stay in contact with Mr. Bo after their bitter breakup in 1981.

The web of entanglements among the families reflects the insular nature of China’s “red nobility.” Ms. Li’s older brother, Li Xiaoxue, is married to Ms. Gu’s older sister, the daughter of an army general.

It was this brother who met last October, weeks before Mr. Heywood’s death, with Mr. Bo in Chongqing.