So my sister is bi, and she dated this girl, then a year later

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So my sister is bi, and she dated this girl, then a year later I start talking to this girl, and we're both pretty into each other, I ask if she wants to be my girlfriend, and at first she's happy, but then she thinks it's a bad idea because of my sister.
What do I do to convince her that it isn't? I think this may be my only chance to get a girlfriend

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