When Leanne Bourne, a mother-of-four from Romford in Essex, began to suspect that her partner Steve  was having an affair, she was heartbroken. But in a twist that she could never have foreseen, the other woman turned out to be her younger sister Larissa Knipe. Now, three years after the affair that tore her family apart, Ms Bourne says she is still struggling to come to terms with what happens - and still has bad days when she can't stop crying. Scroll down for video . Family feud: Larissa Knipe (left) is pictured with her son Logun and sister Leanne Bourne (right) 'I was very protective of Larissa because she was my baby sister,' she says of her happy upbringing. 'She wasn't just my sister. She was like a best friend and I'd do anything for her.' But all that changed when Larissa, who is seven years younger than Ms Bourne, moved into her home when she was 17. Although Larissa says there was nothing between her and Steve to begin with, a night out when she was 20 resulted in a liaison with her sister's partner - and the start of an affair. 'I was 20 when it first started with Steve,' she confesses. 'I went out one night, had a few too many drinks, came back, saw him and we started kissing. It started like that. 'The next morning, I woke up and thought, "What have you done?" I don't know why I carried it on to be honest but I did. After the first few months, I did start getting feelings and began falling for him.' Upset: Leanne Bourne appeared on ITV's This Morning to tell her story . Rapprochement: Larissa is finally winning back her sister after more than three years in the cold . Caught in the middle: Sarah Guyton was caught in the centre of the increasingly acrimonious dispute . Noticing that the pair were spending increasing amounts of time together, Ms Bourne became suspicious but, believing that her sister would do nothing to hurt her, stayed quiet. 'I mentioned it to a couple of friends, to a couple of work colleagues,' she remembers. 'One or two of my friends actually said, "What you're thinking, it's probably feasible". 'Everybody else, including my mum, said, "What are you thinking? That's your sister. She loves you. She would never do that to you".' Then, a few months later, Larissa revealed she was pregnant and claimed not to know who the father was. 'Larissa had never really had a boyfriend. If she had, I'd never met any. Then one day I came home from work and she said she was pregnant,' Ms Bourne recalls. But she was left even more suspicious by her sister and Mr Allsop's reaction. 'Larissa and Steve were both over the moon about it, really happy,' she says. 'I was quite concerned. I really felt like I was going crazy, like I was losing it. I was having a battle in my own mind.' Putting her fears to one side, she supported her sister and was there when Larissa gave birth to her son Logun in August 2011. Speaking on ITV's This Morning, Ms Bourne says she was convinced that her sister was the other woman the moment she saw the newborn. Shock: Leanne also spoke of the moment that she realised her sister's baby had been fathered by her partner . 'I had suspicions anyway but as soon as I saw the baby, I knew because not only did it look like Steve, it look like my children when they were babies. It was just too much for coincidence. 'I had to run out of the room. I saw a friend straight after and I said, "That baby is Steve's". Larissa, who moved out into a council flat of her own shortly afterwards, continued to deny all talk of an affair. But then, consumed with guilt, she finally confessed the truth to their mother Sarah Guyton in a text message that read: 'Well what's she going to do when she finds out Logun is Steve's? I can't hide it any more.' 'I answered the phone and my mum's in a panic,' remembers Ms Bourne. 'She was actually crying, so I said, "Calm down mum, what's happened?" 'And she said, "I'm sorry to tell you but what you've been worrying about is true".' Ms Bourne was, she says, left 'utterly devastated' by the news and furious that despite repeated questioning, both Larissa and Steve had point-blank denied having an affair. Then, a few days after the bombshell hit, Steve moved out of Ms Bourne's home and in with her sister. 'It was a nightmare,' remembers Ms Bourne. 'I did send her some nasty text messages. I called her every name under the sun. I think most people in my position would have.' Larissa responded angrily and more than three years on, the sisters are still estranged and struggling to come to terms with what happened. 'It's still heartbreaking really,' adds Ms Bourne. 'To think we were really close and that's all gone. It's vanished. It's non existent now.'
Leanne Bourne, from Essex, discovered her partner was having an affair . Was horrified when the other woman turned out to be her sister Larissa . Larissa even gave birth to Steve's child in August 2011 . Had initially claimed that she didn't know who the father was . More than three years on,  sisters are still struggling to overcome the affair . Leanne and Larissa appear on My Sister Had My Boyfriend's Baby - and Other Betrayals, tonight at 10pm on Channel 5 .