My best friend's sister keeps hitting on me. What to do...

These past several months every time I'm around my best friend's sister she keeps on trying to flirt with me. I could easily go for her but l don't play along too much with her little flirt activity because of the long and close friendship I've had with her brother. I can recall him surprisingly enough subtly trying to get me to take her somewhere but that was just once. I could totally try for this but am in no way willing to let it happen if it could destroy our friendship. I've been really good friends with him for a very long time so compromising here is really not an option I'm willing to choose but still l could very easily pursue her successfully.

Should I just let this one go or do something about it? I'm just not sure.
My best friend's sister keeps hitting on me. What to do...
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