Answer:
She is shot by Meg Giry- it has been built up from the start of the show Meg is shown to be jealous of Christine's success and fame especially as she's trying to establish herself in Phantasma and get into "Mr Y"'s favour. So when Christine shows up in New York slowly Meg's jealousy and insecurity slowly builds to the climax where she kidnaps Gustave, Christine's son and through the confusion and chaos she accidentally shoots Christine.