Suggestion

So I don't know if I'm right or wrong I'm just sharing the thing on which I'm not able to move on so basically i have read my bro chat with a girl. He is an elder brother and the girl with whom he chat is younger than me so she is telling my brother methods to blackmail me so that he can do whatever he wants like u all are mature so you know the intentions are bad I can't say what he spoke with her but that's wrong they are planning to have 3s** and all other soo what should I do , i haven't Even imagine this in my entire life that my brother is like this he doesn't know that I knew it but I'm in shock i don't know what to do what is right or wrong