Sister's boyfriend and I are attracted to another; harmless or flirting with disaster?

ShutYourMouth
They've been dating for a few months now. Her boyfriend is pretty awesome- good guy and all. There's one thing, and I don't think I've been reading into things too much. He likes me. I know he is attracted to me, it's mutual. None of us are bad on the eyes. Anyway he's made comments and behaves very boyish around me I've noticed. He can't help it exactly, him being the male sex and all.

My sister obviously is peeved. It would be perfectly innocent if she trusted her boyfriend (it's a new relationship) or me (having a slight history of involvement with someone "off limits"). I can tell this hurts her feelings when he doesn't even realize he's blatantly trying to get my attention. I am more aware of how I conduct myself and I do notice involuntarily gestures of attraction (I'll touch my hair and try not to be fazed when sexual jokes/topics come up). It's just a feeling I have and it's like I don't trust myself alone with him. And I don't. I'll admit that I am tempted. In fact I've had fantasies but that's all it will amount to. I won't act upon it.

They got into a fight the other day and walked out on each other. It had to do with him saying something stupid and overzealous about me. I confronted him about it and how it seems harmless but we/he can't be joking like that. Of course he was mature about it and the matter was "resolved" after everyone cooled off (more like swept under the rug if you ask me). There's still this intense sexual energy nobody cares to admit yet everyone is painfully aware of. If these vibes don't sizzle out sooner or later, what do I need to do? Address it with him? Talk to my sister first so as not to go behind her back? Or do my best to disregard him and keep out of their business?

Updates
+1 y
I definitely have a penchant for my sister's guys. I will never go behind her back like that though. It just sucks. I think it has to do with someone I was f***ing. I just started to develop serious romantic feelings for the first time- my heart was all aflutter and I was so bubbly and NAIVE. That went on for a few months and then one day he just told me he was in love with my big sister. Right after we had sex. He just said it. The first person I ever fell for chose my sister over me.
Updates
+1 y
I was heartbroken. I still am. I won't ever understand what she had that I didn't. All I know is it just f***ing hurts.
Sister's boyfriend and I are attracted to another; harmless or flirting with disaster?
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