I flirted with a girl now her brother wants to beat me up?! Plz answer 10pts?

Well basically i was on facebook and a girl i knew for 4years and a girl who i love sent me a message saying: do you want to chill. I said yes and then we were flirtin for days and then her brother found out that i was flirting and he wants to fight me what shall i do? Im 21 and so is the girl i like and her bro is 26. Shall i fight him? Hes sendig me messages that if i dont show up on thursday he will threaten my family. His sister persuaded him not to do anything but he still wants to fight me? Shall i show up or phone the police?
Thanks for your help

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