I expect my daughter to help her brother
Is it because she is older?
Yes, it's her duty as a sister.
It is good for the older one to look out for the youngest. You can teach her some responsibility along the way.
But she is only 9.
Okay, the age of 9 is a bit too young for her to be taking on too much responsibility for her siblings, but she can still help out and learn while she does it. Try to work out situations in which she can handle simple tasks on her own, and you can mostly just observe and supervise.