He sounds very unstable. I would stay away from him. He's being to paranoid about you sleeping with his friend even though its obvious it didn't happen.

He's trying to act like he was just using you for sex but obviously he was interested in being around you. He's just not mature enough to break up with you in a healthy way.

Just don't answer his text. He's only trowing insults at you for attention (again with the not grown up enough thing) acting like a child.

Warn your sister he's bad news and trying to start trouble and that talking to him would be a bad idea. Then cut all kind of contact with him, you don't need someone like that in your life.