Ha, I just read that he is 8 years old! I read this before my coffee... If he listens to his sister, she should not have any problems. Just make sure he agrees to be on his best behavior and she remembers not to be bossy.I would still have a neighbor your daughter can call so that there is someone close by till you can get there in case of emergency.
You know your daughter the best. If you feel she is a responsible child then also consider....If you feel she is calm under pressure and can easily get in touch with you, I am sure she will be fine. Do you already leave her with her brother for short periods iof the day while you run errands?
Has your daughter ever baby sat for anyone else? Does she know how to care for your son? Diapers, feeding, sleep routine?
Has she taken any sort of CPR or First Aid class?
If you feel she can handle all of this, the only other thing I would suggest is that you also let a neighbor know that this will be the first time for your daughter to "Officially" babysit her brother, so would that neighbor be willing to be called in case of emergency. This way there is a neighbor very close by as well as your daughter able to call you or 911 in an extreme situation.
I am sure she is going to great. This is going to give her such self confidence and could be the beginning of a very profitable, job for your daughter.
Our daughter has baby sat since she was this age and really has a following, but our daughter would only watch toddlers. She was not comfortable with children any younger. A child that can watch a baby is a great find..