"But sometimes, nightmares are real."
For as long as Clary could remember she was always plagued by these awful nightmares, ones she couldn't pull out of until it was over. She'd have to sit through it all, feel the persons life slipping away, listen as they cried for help. But that was never the worse of it, the truth was.
They were never her dreams, they belonged to her sister - she couldn't understand it though, how she could see her sisters nightmares, but she could. But after seven years of dealing with it, she left her father and sister and resumed her job as a hunter who killed the supernatural.
But she had never realised how strong she and her sisters bond was and those nightmares would never go away. And eventually, they started trying to help, to see if what was really happening in their dreams was real. And it was.
Which was why her sister, Olivia, was so freaked out when they both had a dream of her old friend Sam Winchester dying. So what could they do? Get themselves wrapped up in whatever mess the Winchesters had created? Get themselves turned into wanted fugitives by the end of the year? Well, that's exactly what they did.