Girls Aloud sought Sarah Harding's mother's blessing before deciding to reunite.
The group will embark on a tour of the UK and Ireland next summer to mark Girls Aloud's 21st anniversary but Cheryl Tweedy, Kimberley Walsh, Nicola Roberts and Nadine Coyle refused to go ahead until they secured Marie Hardman's blessing.
Sarah died two years ago at the age of 39, following a battle with breast cancer.
A source told the Daily Mail newspaper: "The girls wanted to include Marie from start to finish. They wanted her to feel part of all of this and not do anything at all that might not be right for her.
"Marie, of course, was absolutely thrilled that there would be a tour, but also that Sarah will be very much included in it to the extent that nobody thought would be imaginable.
"It was important to the girls that they ran through everything with Marie. They love Sarah so much, it wasn't an option not to include her in the reunion because she was one of theirs and she loved the band so much. There was sheer jubilation on both sides. Nobody wants to forget Sarah."
The group have invested in new technology which will allow them to mix Sarah's vocals with their own during the tour.
Footage and photographs of Sarah will also be included in the concerts, while the profits will be equally shared with Sarah's estate.
The insider added: "That's the right thing to do. There were five girls in the band."
The band first found fame in 2002 on TV show 'Popstars: The Rivals' before becoming one of the most successful female bands in British history.
The band split in 2013.
