You don't think it's a good idea she knows someone she's not good with has the power to light things on fire?
Luke, I know shit is hard - and Wren acts crazy at least thirty-three percent of the time, but don't you think she ought to hear it from you before she hears it from me? Or finds out from another source? Or worse, first hand? You can't stop her from doing crazy shit, so accept that and find a way to tell her that will tone down, rather than cancel out the crazy.
I'm not saying you gotta tell her all the gorey details. Just the basics, keep her safe if that's a concern. You gotta tell her. Today.