what do you think!!!

Even as I guy I think the whole episode is pretty classless. Even if it was his sister, he could have just said so. However, heading to the bathroom pretty well indicates otherwise. Personally, I would be asking why he would even talk to her at all while in your home. Seems pretty odd to me. I think I would be tempted to approach him about it and find out what the heck he is up to.

Let him know how you feel. Don't let your decision of seeing him any more be based solely on that, it may be no big deal. However, a relationship should be open and honest, so it could be a sign to the contrary. Just be honest with him, let him know how you feel about it.

I have been in that situation before. It turned out that the person hewas talking to was his sister. Later, I got to meet her. You may not be in the same situation. You should atleast find out who this person is to him first. It's not fair to him to end a relationship because you got jealous until it is justifiable. Maybe he deserves the chance to tell you something that would make you not want to end the relationship. I would atleast ask, if I were you. It isn't being nosey unless he has something to hide. In that case, I would end it. Just be casual about it, sometimes a little harmless jealousy is okay. Ask him, who was that? like you don't really care what the answer is, that way you don't come off accusing him yet.

Have you asked him about it? It might not be what you think.