I had a friedn in my teens who was told, aged 15, that her 'mum' was in fact her gran, and her sister (15 years older) was in fact her mother. The sister had since married and had two more daughters who she kept.
Within months of finding this out my freind was pregnant herslef. Obviously her Mum/Gran was much older and just had no appetite to go through it all again, and forced her to leave home. She lived in absolutely grotty accommadation, but I suppose at the time, to her it was all a big adventure.
Eventually it stopped being an adventure and started being a tough, miserable existence. She started a college course but left after two terms because she couldn't cope. Then when she was about 18 she got PG again, and had a very late abortion (traumatic) because she recognised it would just make her already quite shit life even more shit.
She was a very bright girl as well. But I used to stand and watch with horror and bewilderment as she's scream ans spit with rage inot her two year old DD face, and call her a fucking bitch and a cunt. That is what being poor, alone and depressed and desperate does to people. 
But still all these young girls think it won't apply to them.