Here's that question

I'm guessing the question here is "what should I do?" and the answer, I think, is: Nothing, for now. ..
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He's off-limits as long as he's your sister's boyfriend, even if she doesn't really like him. (that's not exactly *fair* to you, but tough luck -- it's your sister. special rules apply.) ..
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If she breaks up with him, you should ask her if it's okay for you to date him. And if she says yes, you have the green light...

For now, you can't make a move. She is your sister and theres a rule there and you can't interfere...If she's not that interested anyways, maybe they will break up soon. These odds are especially great if she is seeing another boy too.....
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Once they break up, you can explain your feelings and she shoudl give you the go ahead if her feelings were never strong for him anyways...

If you don't want to end up on a daytime talk show throwing chairs at each other -- I'd leave the guy alone...

I'm here to offer a guy's perspective...
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Maybe I'm just too simple here, but why don't you tell her you'd gladly take him off her hands. Your relationship with her is more important than any guy, but if she's not really into him, as a good sis she might want to help you with your problem, benefits her too...
-Carlos..

It unanimous ... seems like everyone here is of the opinion ... it's a no go zone for now ... But I couldn't help but add a thought I wouldn't want to go unmentioned .....
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By the sounds of it u and ur sister have a very close relationship (for her to have confided in you about her feelings) .... Preserve and protect that at all costs ... u'll see u'll come to appreciate, rely and count your blessings that you have her in your court ... she can be your biggest supporter, best unconditional friend and one of the people you will cherish as you "bump along" all thru this life.....
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When the time is right, confide in her about ur feelings for him and likely, if she's in tune, she'll probably have picked up on it already ... but she'll have an actual lesson of how much she can trust you ... ur bond will grow stronger because of it.....
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Bonus! with her "in the know"..... if things DO work out with this guy, you won't be starting off with him harbouring all kinds of guilty feelings...Angie..