Haleema Butt and her husband, Usman Darr, were suspended soon after the attack on June 4 and questioned by security services.
Haleema, a 28-year-old mother of two, has now had her employment terminated, according to the Daily Mirror.
"Haleema and Usman are not extreme at all. They're really upset about what Khuram did," a source close to the couple told the newspaper.
"You should take it up with Heathrow," Haleema Butt said when asked about her job at her north-west London home this week.
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Usman, who has worked at Heathrow airport for a number of years, is believed to be still suspended.
While there is no suggestion that Haleema Butt or Usman Darr are extremists or knew what Khuram Butt was planning, Haleema is believed to have had her contract terminated following a probe over staff vetting procedures.
Khuram Butt, believed to be the ringleader of the terrorists who crashed a van into pedestrians on London Bridge before going on a stabbing spree in nearby Borough Market claiming eight lives, was known to the UK's M15 intelligence services as a key figure in the extremist group Al-Muhajiroun.
UK terror experts are now calling for more stringent background checks on people applying for sensitive jobs.
"Security roles at places such as Heathrow are critical - these are the people stopping terrorists putting bombs on planes. We need to be doing more to check the names of employees against terror watch lists," Col Hamish de Bretton Gordon, former head of chemical weapons for NATO and the British Army, said.
"We can't suggest this sister is in any way sympathetic to her brother's views, but there have been enough terror cases where relatives have been acting together for the need for these checks to be done.