VENNNNTTTTTTT

So my boyfriend has a brother who has a girlfriend of over one year and my friend has been recently getting extremely close with him. Not only does it make me uncomfortable because that's my mans brother but also he has a girlfriend!!!!!! Why can't girls understand that it is not okay to talk to and flirt with taken men!?!?! It makes me sick to my stomach knowing she is okay with herself to do that. Who's next? My boyfriend??? I can't explain enough how much upsets me.... please calm me down.