If you want your sister's friend to stay away from you, just tell her. Otherwise tell your sister about it and ask her whether it would be ok with her if you were with her friend.
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It's important to set boundaries and make it clear that you are not interested in a romantic or intimate relationship with your little sister's best friend. Communicate this in a respectful and firm manner to avoid any misunderstandings. It may also be helpful to talk to your little sister about the situation and seek guidance on how to handle it appropriately while maintaining the relationships.