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 did n'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 10 pm on channel 5 .