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If it is a short walk and she is responsible then there is no reason why she can not walk her sister home from her after school club. Many parents allow older children to escort younger children home from school and after school programs. As long as you are comfortable with your daughters ability to go straight home and take care of her sister on the way there is no reason why she can not help out in this way.
You may want to consider picking up one of those prepaid cell phones for her to have in the case of emergency. Even though its a short walk, it never hurts to for her to have a way to communicate with you should she need to.
Just make sure that you inform the school that this is the way she will be going home in the future.