The two remained in detention for more than a year before they were released on bail in October 2018. She faced 22 years in prison if convicted.
The Rwandan election commission had said that some of the 600 signatures she had submitted - a requirement for aspiring presidential candidates - were forged, and that some of the people on her list were dead.
Since her arrest, Ms Rwigara's family have been subject to interrogations and their family assets forcibly auctioned.
"Diane and Adeline Rwigara should never have faced charges for expressing their views". But the court ruled that these were merely conversations between individuals and did not prove that she was promoting insurrection. She - just like Ms Rwigara - had meant to run for president against Paul Kagame, back in 2010.
"I am very happy with the verdict", Rwigara told journalists after the high court ruling on Thursday. Everything I talked about in the past has not been resolved.
Assinapol was an influential industrialist who played an important role in financing the ruling Rwandan Patriotic Front (FPR) in the 1990s before later falling out with its leaders. "The judgement must be a first step in reversing the ongoing trend of repression in Rwanda". "While we welcome their discharge and acquittal, we are concerned that the right to freedom of expression remains under attack in Rwanda", Amnesty International's Director for East Africa, the Horn and the Great Lakes, Joan Nyanyuki said.
The three-judge panel said there was no proof that Rwandans had been incited against the state after Rwigara's remarks to the media, and that intercepted WhatsApp audio files of her mother did not incite insurrection and instead were private conversations.
Kagame is praised for leading Rwanda's recovery from the 1994 genocide and for advances in economic development and women's rights, but critics say he does not tolerate criticism.
Four other accused relatives, who are not in the country, were also cleared of all charges and found not guilty.
Opposition leader Victoire Ingabire was released from prison this year having served time for "conspiracy against the country through terrorism and war" and "genocide denial".
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In March, the Rwanda Revenue Authority sold Premier Tobacco Co., a firm owned by the Rwigara family, after it failed to pay $6 million in taxes and debt to banks.
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