Getting jiggy with your ex's bro?

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What started as a friendly 'hi' to my ex's brother turned to be a semi-fling. I just wanted to be friends with my ex's circle, no motives, no hidden agenda. And the ex of course doesnt know it..kinda behind his back (Im half scared of what he'll think of me and half wanting to hurt him). I dont want to add any more meaning to this since I have NO plans of even dating the brother. Since my ex did some behind my back of his own as well, (im not really out to do the same, but it just happened -- the brother is suddenly flirting with me.) I somehow feel justified? I know i'm not making sense, but what's your take on this? A friend told me it's a no-no, another told me to 'go with the flow' so i'd be distracted and also get even. Is this a top of the list off-limits?