I'll tell you what, the whole notion of a sister acting that way with her brother is disturbing indeed. But look, if that email led you to such decisive action even in the face of his arguments that he did nothing wrong tells me that there were many problems with this marriage even before the email was discovered. In healthy marriages, the wife would have absolutely been shocked and devastated, but if given a chance to explain/defend himself, the truth would have come out and repair could have been possible. It sounds as though this email was the straw that broke the camel's back, and that you were finally able to leave a bad situation.
I applaud your entering counseling, as you can absolutely learn to build better boundaries and to communicate better with the people on your life who are closest to you without feeling as though your needs/concerns are being ignored.
If you are satisfied with the response, please hit "Accept." That is the only way I can receive credit for my answer. Thanks-
Wow... a lot of history there. Take care of yourself, okay? I wish you the best-