The men were each sentenced to just over 76 years in prison. Three other men were convicted of involvement.
The murder of Marielle Franco, a member of the Rio de Janeiro city council, drew widespread attention. She was a prominent human rights activist and a sharp critic of police brutality.
According to the court, the brothers had connections to the militia groups that control parts of Rio and the purpose of the murder was to send a "message" to the city's political elite.
Franco was 38 years old when she and her driver were shot dead in March 2018.
Two former military police officers were sentenced to long prison terms in 2024 for carrying out the killings.
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