How are you presenting it? Not sure if you have access to presenting it but heres how to present it form sheet.
let viewController = LoginViewController()
viewController.modalPresentationStyle = .FormSheet
self.presentViewController(viewController, animated: true, completion: nil)
I think you can also have the sign in VC show the login view controller (i.e. it slides from the side) if your sign in view controller is embedded or part of a UINavigationController by doing:
self.showViewController(viewController, sender: self)
