family madness

So I'm 28 currently 14 weeks pregnant with twins... My older sister and I don't get along so we don't communicate anymore. She has a 18 year old daughter who now has a problem with me! Mind you I don't say anything to these people....i use social media to tweet just for the hell of it nothing major just RANDOM tweets. Well the 18 year old must have seen an innocent tweet that I posted I don't know but I can assure you that it had absolutely nothing to do with her && she got upset and started tweeting me saying she is coming to my house tonight to beat my ass and things like that. Now Before I have had my share of fights but I'm married now with children on the way! && I'm not going to be trashy and fight my teenage niece anyway... But if she does come to my doorstep tonight how should I handle it?? I'm scared of what I will do to her if I get angry enough!!