The Irish girl group, who famously scored four No.1 singles with their first four tracks, are among the acts reuniting for the TV show and spilling the beans about what really went on when they were at the height of their fame.
[B*Witched in 2013]
Twins Edele and Keavy and friends Sinead and Lindsey confess in next week's show that there were problems with the group from the outset because of jealousy towards lead singer Edele.
Sinead said: "[Edele] was definitely the spokesperson, the leader, the lead singer, she was definitely the one. [The other band members] kind of felt what's the point of being in this situation, being in this band if we're not contributing to it?
"It must be really difficult watching your sister, your twin, knowing that the two of you sing very similar and seeing her being out there. Keavy found it really hard to deal with that."
Keavy added: "I was definitely sad about it, I very much felt like I was in my twin's shadow which is why I was the dancer, why I was the joker because I just wanted to be seen. I wanted to make sure that people actually knew there was two of us with the same face."
Watch our interview with the Big Reunion stars:
Meanwhile, the success of the group and the world tours were having a negative effect on Keavy's health.
"I didn't know how to reach out, I didn't know how to tell someone that things weren't okay. We had a week's holiday and I went to Portugal with my family and my best friends and that's when I really started to notice something was really wrong," said the singer.
"I just went, 'I can't do it anymore'. I phoned Edele from my hotel room, I couldn't even talk to her. I was sobbing, I just lost it. Edele said to me, 'You need help'. It was like a relief that she said it to me."
Edele admitted that she felt guilty about his sister's emotional state and said that the record company's reaction made her realise how badly they were being treated.
"I rang everybody and I said, 'Keavy is going away for three weeks, end of story - she's gone and I don't care what any of you say, what matters is that Keavy is shattered right now'," said Edele.
"To these people we were a product and their product was being tampered with as far as they were concerned."
The Big Reunion airs on Thursdays at 9pm on ITV2.