Please be aware that all the information concerning pregnancy and childbirth may not be correct, but it is how I need it for the plot to work. Enjoy!
Jace looked as if his head was about to explode. Splotches of red dotted his cheeks and his eyes were budging so far out of his head that they might have fallen out had Clary not put her hand on his knee.
"This can't be true… It isn't possible. Valentine cut me out of my mother himself. Surely he would've noticed if there was another baby!" But, whilst Jace was lost for words, Jessica was ready and waiting with an answer.
"Ah, see, that's where you and Valentine were wrong. So, when a woman is pregnant with twins, there is a divide in the uterus, separating the babies. So, when Valentine went to cut you out of our mother, the divide was still in place. He didn't see me. You know he took Hodge Starkweather with him? Well, as they were walking away from our mother's body, Valentine holding you," she gestured toward Jace, "and Hodge following behind, Hodge heard the cry of a new-born. At first, he thought it was you, but Valentine was striding ahead, and you were out of earshot. So, Hodge turned back to our mother's body and retrieved me. Why he didn't give me to Valentine, I'm not sure. But he took me to the London Institute and let them raise me."
As Jace struggled to comprehend all the information, Clary raised her hand.
"I also have a few questions. One, my mother is still alive and Valentine wasn't around when she gave birth, so, how could I have a twin and not know about it? Two," she turned to Jessica, "how do we know you are telling the truth? For all we know, you could just want to use us to climb your way to power. What reason do we have to believe you?" Jessica and Connor, who hadn't spoken much, looked at each other. Connor took a deep breath and started to explain his story.
"Well, when Jocelyn found out she was pregnant with you," he gestured vaguely at Clary, "and found out what Valentine was doing to you, she ran. But, what she didn't know was that she was pregnant with twins. So, when she finally gave birth to you, she thought that was it. She was safe. End of story. But, unfortunately for her, about twenty minutes later, she felt contractions again. After she gave birth to me, she made the decision that we should be kept separate as to best protect us. Why she decided to keep you and abandon me, cry over her demonic first born, and yet never even feel any sadness on our birthday, I'm not sure. But, you wanted to know why you didn't know that you had a twin, there you go. Jocelyn may have a completely different story to mine, but that is what the Institute and the High Warlock of London told me. Also, don't ask me why she decided to raise you as a mundane and give me to the London Institute, so, please don't ask." Connor glanced sideways at Jessica, who was gazing absentmindedly at the window. He punched her lightly on the arm to snap her out of her daydream. She started and looked up suddenly.
"But that's not the only reason we crossed the Atlantic. Something's coming to New York. Something bad. Worse than Lilith or Jonathan or Valentine. It's something big and this time you might not be able to stop it."
