Have you tried talking to her very nicely?
Children are not always good at reading "signs" because they are not socially developed. If you push her away emphatically, you can hurt her self esteem. Clearly she doesn't have a mother figure to connect with and she sees you as an older sister. Talk to her. Tell her that while you enjoy her company, you have things to attend to. You have a baby that needs to sleep in order to grow. Tell her she can visit you every other day for an hour or something. Take control firmly but gently.
Also she could be bored because she may have no hobby. Perhaps you could talk to her guardians about keeping her occupied somehow. A musical instrument or a craft would help. Or afterschool activities.
I realize that it seems like too much effort for someone you are not related to, but this little girl doesn't seem to know any better. She has connected with you for one reason or other. It could really hurt her in the long run if you snap at her for her behavior. Of course, this is just my opinion.
Good luck.
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