My daughter is now 11 years old, but still asks me to zip her jacket. This really worries me because she can do it herself and is otherwise independent. She even asks me when her friends are around. Her 3-year-old sister zips her jacket for her, which proves to me that she should be zipping her own jacket. Usually, I say no, but if her hands are full or I am in a rush then yes.
It might be that your daughter is missing being the "baby" and having things done for her. This might be her way of asking if you love her as much as you love her little sister, who gets her jacket zipped and lots of other things that your daughter used to and doesn't any more. It's hard to grow up, and I'm sure that even adults occasionally look back with wistfulness to a time when we were nurtured and cared for by our mothers.
As long as the behavior doesn't escalate to others, I wouldn't worry. But do remember that although it's easy to start relying on an older child to be independent when you have a younger who needs more care, she is still a child who needs to know that her mother loves her. I'm sure that she does know that, but we all need a little reassurance every now and then.