Barbara Henning, who made a tearful plea for her husband's release before his brutal death, has been living with his brother-in-law since January, the aid worker's sister claimed today.
The pair had been having an affair since before Mr Henning, a taxi driver from Eccles, Greater Manchester, travelled to Syria last December, according to his sister, Gill Kenyon.
Mrs Kenyon, 50, decided not to tell her brother about the affair before he left for the war-torn country - a decision which she says has 'tortured' her since.
'I never told Alan before he went. It has tortured me ever since,' she told the Sun.
'He will have had so long to ponder things and what would he have had to come back to?' Mrs Kenyon told the Sun.
Mr Henning was barbarically murdered on October 3 at the hand of 'Jihadi John', a hooded militant responsible for the deaths of several Western journalists and aid workers.
The 47-year-old had travelled to Syria to deliver aid to children in the region whose lives have been ravaged by years of civil unrest.
In January this year, just weeks after arriving in the country, Mr Henning was taken hostage by ISIS militants.
Along with other Westerners, some of whom were released, he was held in captivity before being paraded in an orange jumpsuit and murdered in one of the organisation's chilling videos.
His death sparked an outpouring of sympathy and outrage, prompting American and their allies forces to increase airstrikes across the Middle East.
Now, almost one year since he was plucked from the remaining volunteers offering aid in the country by fanatics, Mr Henning's sister tells of how she couldn't bear participating in vigils held in his memory as she would run in to his adulterous wife and her husband.
'I needed to grieve for my brother but I couldn't go to the vigils because Pat and Barbara were together and I was grieving for my marriage.
'The pain of what I have been through is just indescribable. I've lost my brother and my husband.'
Last night Barbara Henning did not deny she had been having an affair, but did not clarify whether her husband was aware of the relationship.
'Everyone who needed to know knew about us. This started way before Alan went to Syria,' she told the newspaper.
Earlier this year Mrs Henning led mourners at a joint service held in memory of her husband and fellow Brit David Haines, killed by the same fanatical group in Syria.
With her daughter Lucy, Mrs Henning linked arms with the family of Mr Haines as crowds gathered to pay their respects to the pair.
