My sister's boyfriend likes to flirt with me a lot.

My sister's (he’s 17 my age and she's 14) boyfriend likes to flirt with me a lot. He told me a few times he doesn’t want my sister and rather have me instead. I do think he’s cute but I don’t want my sister to stop trusting me. My sister doesn’t know about this and he’s almost her first boyfriend. Any ideas?
My sister's boyfriend likes to flirt with me a lot.
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