I think there are 3 points you need to consider:
I don't think you can tell her. For the simple reason anything your sister might or might not say to you doesn't have any real meaning. Aside from the ethics I can say every day I can kick my kids out. But until I actually do that it's just talk.
There also is a legal side to this. In many countries a 14 year old cannot just go and live anywhere. They need to live with their legal guardian. Is your sister willing to sign the paperwork?
Talk to the authorities. Social services isn't a child stealing organization. If you have a place to offer for your niece they quite likely would help you to make it happen.
