Thirteen years old means extreme maturity in one adolescent and complete immaturity in another - so I don't know if you are expecting too much of this child or not. But let's assume your daughter is average and could be expected to meet the challenge of this task. It is important for you to set her up for success. Younger siblings don't view the eldest as a real adult or a substitute parent. You must empower your daughter by letting the younger ones know that any negative report that is received about them will definitely result in punishment. You will be accepting the older one's word about the situation. Take the older one aside and explain that if she were to exaggerate or give a false report even once, she would loose her credibility with her sisters and with you for years to come. Tell her that you trust her judgment. Let her know that babysitting is a big responsibility and that she needs to be "professionally" prepared for it - she should read a babysitter's guide and/or take a babysitting course. You will give her a little booklet with important numbers and information, just as you would do for a "real" babysitter. And, you should pay her for her services - since this will communicate that you see her as an adult who is doing an important job. (You could barter instead, offering products and services in exchange for babysitting).When you elevate the position and the person holding it, the siblings will have increased respect for both as well.