Only you can provide private information to your sister in law. If she persists knowing something you do not want to tell her then bluntly tell her that you would rather not discuss it. Tell her what you want too because generally nosy people will gossip about it and anything you want to be kept private do so. When anyone is persistant about something you do not want them to know you have a right to be blunt and to the point and do not worry about upsetting that person. Loose lips sink ships!