I would treat her the same as all of your other employees. Trust her, respect her and speak to her professionally. If it turns out she has a problem with you speak to her privately about the situation if it arises. She may well be jealous that you are now above her but at the end of the day you have worked hard and earned your position. You did not undercut anyone to get where you are or stab anyone in the back. Be a happy boss, not one which goes round being miserable and this will help to keep your employees spirits high but at the same time if she does rebel from what you are doing you will have to treat her just like anybody else. If you start to treat her any differently from your other employees you will have a problem with them on your hands too saying that you are discriminating against them.