Alec Lightwood/oc Clary Fray has a sister, Celestine Fray. Now imagine a few conditions that can result in her and Alec Lightwood getting married even though they barely know each other. And then imagine a messed up marriage, everyone finding out that the Fray twins are Valentine's daughters. Now throw in the Lightwood family and the clave and add a love triangle. Wait. 2 love triangles. Then shove in downworlders and exes along with weird friends. There, you have Celestine Fray's existence.