Is what my brother doing wrong?

If it bothers her and hurts her feelings, then yes it is wrong. Not only wrong, but inconsiderate. Just walk up to him (kind of like you know something and thinks he has the right to know also.) Then ask him this: "If you found out that your girlfriend was talking to her ex and hanging out with him, would you be upset?" And when he says 'yes' (uming he will), then let him know that she isn't. But that it really hurts her feelings that he does. good luck gigi