If you had your sister talk to him, would it help? Trust is a very difficult thing to earn, and even more difficult to gain back once it has been lost. If you have already explained to him what the situation was, then it is kindof up to him. He has the choice of whether or not to believe you. If he refuses to listen to what you or your sister have to say about it, then there really isn't much you can do. But, if you haven't broken up, as of now, then it may be affective if YOU break up with HIM. Tell him you can't be with a person that doesn't trust you. At that point he may realise that you are serious about not "cheating" on him, and he may appologize for not trusting you. But, there is the chance he will think you are breaking up because you want to be with this other person. If that happens, then you should just give it some time, and when he figures out that he messed up, he will come back to you. Good luck with it, I hope you get what you want.
