A couple of guys my sister liked ended up liking her instead, she may resent her for it. I was gonna talk to my sister more about it, but she didn't seem to be in the mood at the time. It could be more reasons, who knows?
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maybe your sister feels like you having a girlfriend will distance you from her. or if she's younger maybe she ñooks up to you and doesn't want to lose her role model
I think my sister might be jealous that guys find her hot, a couple of guys she tried dating ended up liking her.