She said he cheated on me because of a comment that was targeted to her brother who is a jealous type and we all knew it was a joke even her?

My boyfriend (19)and her brother are best mates. The is 16 and is hugely obsessed with him, going on 2017 pictures commenting “good looking❤️“ and he doesn’t flirt back. She loves him to the core and won’t respect my relationship! All of this started when my boyfriend decided to play a joke on her jealous brother. We were ALL AWARE OF THE JOKE INCLUDING HER!! Obviously I was upset about it so we argued and the girl said sorry to be the cause of the arguement and her brother took the mick out of him because my bf got karma for joking with him lol. However, a few days ago when she went on the 2017 picture, I asked her to not call him good looking or send hearts to him anymore, i also said it will be the last time that I will be nice to her because of the friendship between her brother and my bf. Yesterday she say the messages and fot very defensive and aggressive, calling me ugly and all but she knows I’m prettier than her, even my bf and her brother claim so. And she also said he cheated on me because of that joking comment. She tried to hurt me by saying something that is not true, she is a very aggressive girl for her age. I asked her to respect my relationship since she follows christianity just like she claims lol. I also said that I trust him and that i spend every day talking to him. How do I manage this girl?