"I was clueless," Helena Benaouda said in an interview with daily Dagens Nyheter (DN), speaking publicly for the first time since the arrest last month of son-in-law Munir Awad.
Awad, a 29-year-old Swede born in Lebanon, is currently detained -- along with three other men, including two Swedish citizens -- on suspicion of planning a December attack on the Jyllands-Posten daily which had published caricatures of the prophet Mohammed.
He had been twice detained abroad suspected of terror links -- once in Somalia in 2007 and in Pakistan in 2009 -- both times with his wife, Benaouda's daughter Safia.
Benaouda, who previously said she had never come across any Muslim extremists, said it was possible her daughter did not know of the plot.
But she added she should have been suspicious about her son-in-law's plans.
"Safia says 'I don't get it and I don't know what he is up to'. And I should have known myself. How is it possible to hide such things to those close to you?," she told the paper.
Awad had also shared a Stockholm-area flat with one of two Swedes of Somali origin who were sent to jail in December for "planning terrorist crimes" in Somalia.
When Awad's former flatmate was arrested in June, Benaouda told her son-in-law "that he shouldn't mix with people who get arrested. But I didn't say more, because I don't have such a close relationship to him", she told DN.
Benaouda said she had received threats and not publicly spoken about her son-in-law's arrest to take the time to deal with the family crisis.
She condemned all forms of extremism in Islam, including Sweden's first suicide attack, carried out in December by an Iraqi-born Swede who killed only himself after sending a message saying he was acting in the name of Islam.
"There are no holy wars in Islam," Benaouda said.
"For me, the Stockholm suicide bomber is a criminal. I distance myself from all Islamic extremism. The use of violence is always unacceptable."