It sounds like you feel like your sister is trying to hurt you by "taking" your best friend. I can understand why you might feel hurt and left out, but I have the sense that no one was trying to upset you. Having a friend is not like having a bike or a cell phone. Those are things that can be lost or taken. But a friend is a human being with interests and ideas and feelings of her own. And in this case, it sounds like your best friend has chosen to spend (at least some of her time) with your older sister. She has that right in the same way that you can choose to be annoyed or upset about it. I suggest you talk with your best friend about how you've been feeling. If it's at all possible, maybe the two of you can make plans to spend time together, without your sister. That way you can reconnect with your friend and figure out how to strengthen this friendship, moving forward. I hope this helps.