The mother-in-law and brother-in-law of Sayed al-Wadaei were both recently detained by Bahraini authorities, his wife told Human Rights Watch, following what she said were continued threats against the activist's family.
"Where shall I go first, shall I go to his family or your family?" a senior official allegedly told Wadaei's wife at Bahrain airport.
He also referred to Duaa al-Wadaei's husband as "an animal".
It follows years of campaigning by the London-based activist against the Bahraini government, including the ruling al-Khalifa dynasty. Other family members still living on the island have also been previously arrested.
"This looks like a cowardly attempt to break the resolve of an activist by attacking his family," said Eric Goldstein, deputy Middle East director at Human Rights Watch.
"The Bahraini authorities forced Sayed al-Wadaei into exile in Britain, where he's a thorn in their side. Since they can't touch him, they've resorted to threatening and harassing his wife, infant son, and in-laws."
Harassment of the families of exiled critics has been a widely reported abuse in Gulf states. But accusations by Duaa are particularly alarming as one of these incidents involved an infant.
Human Rights Watch reported that in 2016, Duaa was checking in for a flight to London with her two-year-old child when she was allegedly approached by senior Bahraini officials who took her to a private room.
She was made to remove her headscarf and shoes, while her bags and the baby's stroller were searched. When she refused to move to another part of the airport, Duaa says she was dragged by her wrists and separated from her son.
After a seven-hour interrogation, Duaa was allegedly told "Deliver this message to your husband - I will get him" by one of the officials.
Sayed has been granted leave to remain in the UK, after he fled Bahrain in 2012, when he said he was detained and tortured by police.
He set up the Bahrain Institute for Rights and Democracy the following year, which has worked to highlight human rights abuses by Manama.
Bahrain erupted in protest in 2011, when anti-government, demonstrations led by the island's Shia-majority swept through the country.
A Gulf-led police force put down the protests, but the GCC security force has not managed to quell unrest in Shia villages who complain of discrimination by the Sunni-dominated government.
Manama meanwhile says its security forces had come under attack from "foreign-backed" militants.