Harrasment from an adult against a minor?

So heres the lay down. My best friends ex-boyfriend is crazy. I have been best friends with her for 5 years and they had been dating for 4 months. Every time we hung out he would get jealous and text me maliciously. Usually reffering to me being gay which im not. This has been going on with lots of other people too over the years and its never in a nice way and has caused physcological issues. On saturday I was eating at a restaurant with my best friend when the boyfriend called and guessed she was hanging out with me and ofcourse got jealous. I then received a text message from the boyfriends 18 year old sister saying "Hey Fag, I swear to god if another incident happens like this with my brother involving you, there will be hell to pay. " I have honestly never even seen her and talked to her before at all. I was then threatened that she was going to come to the restaurant that we were at and " back her sh!t up ". I then left immediately to refrain from conflict. And ofcourse it was just stupid mean texts back and forth between me and her. I tried adding her on facebook, and she messaged me pretty much bashing me. Later that day " today " their mom called me and told me that she had reported me to the police and filed against me for harrasment. And I told her to " Go ahead " and just hung up because this mother I had never talked to or seen before started screaming at me. My older sister is aware of this and knows the girl and definetly does not like her, and then called her and left a voicemail telling her that if she had ever talked to me again she will not hesitate to report her, because my sister is aware of m physcological issues. Then tonight I was getting the same stuff from the girls friends. Calling me names, telling me to have fun in hell. I told one girl in particular that was saying this to tell the boyfriends sister that If I ever get contacted again, talked to, or even flipped off on the road or something I will not hesitate to take her to court and press charges for Psychological Harassment against a minor. What actions could I take? I could have called the cops at the restaurant but didnt want to because I was saying mean things back, which I know I shouldn't have and didn't want to get in trouble. Im pretty aware of the law. Just please tell me what to do!!