Sayed Alwadaei's brother-in-law, Sayed Nizar Alwadaei, his mother-in-law, Hajer Mansoor Hassan, and a cousin, Mahmood Marzooq Mansoor, were convicted on charges of planting fake bombs in an area southwest of Manama after a grossly unfair trial in which they say they were tortured to "confess".
The trial has come under worldwide scrutiny as a case of possible retribution because of BIRD's activities.
Human Rights Watch also condemned Monday's verdicts.
"Today's guilty verdict on dubious charges against three relatives of a human rights defender are testimony to Bahrain's comprehensive campaign to muzzle dissent", said Joe Stork, HRW's deputy director for Middle East.
Last month, Amnesty reported on the Bahraini authorities' track record of harassing, threatening and detaining family members of Bahraini activists living overseas as a means of intimidating them into silence.
On October 26, 15 non-governmental organisations, including Amnesty International and Reporters Without Borders, issued an open letter to the governments of Canada, France, Britain and the United States, among others.
Hajar Mansoor Hassan, one of the five detainees, had been forced to suspend her hunger strike at the weekend after being hospitalised for low blood sugar levels.
Authorities have since agreed to their demands, which include clean sheets, privacy during phone calls to family and removal of a glass barrier during family visits.
A key US ally and home to the US Fifth Fleet, Bahrain has tightened its grip on dissent since 2011, drawing harsh criticism from global rights groups for its treatment of the Shiite-majority population.
Bahraini authorities last week released high-profile female activist Ebtisam al-Saegh as she awaits trial.
Dozens of Bahrainis have been jailed and stripped of citizenship since the 2011 outbreak of protests demanding an elected government in the Sunni-ruled kingdom.
Bahrain accuses Iran of training "terrorist cells" that aim to overthrow its government, an allegation Tehran denies.