I don't use Parental Controls but my understanding is that while you can do things such as limit apps use and various settings, you don't have full control of the account. If you know your son's Apple ID and password, read Check your Apple Account device list to find where you’re signed in - Apple Support about removing a device from the list and permanently signing out of it. Do this on the web.
Your sister made a big mistake in letting another person log into her device. It basically gave your son ownership of the device.