The character (かがみ) is asking her sister to explain something, so she points to the textbook and says:「ここなんだけど」. It's translated (presumably correctly?) as: "I'm talking about this". (The sister responds with 「かがみが わからないところは 私に分かるわけないでしょう」.)

What is the role of the particle here? I thought would reflect some feelings or thoughts that are (expected to be) shared between the two people. But this meaning doesn't seem to fit here.

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