Everything you've told us about their relationship and interactions--everything you believe to be true--is based soley on what they have told you. You haven't independently verified anything, and you aren't going to verify anything with a polygraph. Correct? WH kept his feelings a secret from you for 5 years, and SIL kept her interactions with WH a secret from you since March. In order to keep this from you for such a long period of time, they must have entrenched habits of lying and/or withholding information from you. What are the odds that they simply broke those habits the moment you discovered this? And if you believe that your husband has some sort of mental illness, that is even more reason why you shouldn't simply believe what he tells you.

As far as I am concerned an EA is an EA, whether it is one sided or not. I would expose it far and wide. Start there.

CV

Please read this and listen to the clips.
What is Just Compensation?

You may not realize it yet, but you are getting good advice. I hope you stick around.

The first rule is to stop believing everything your WH tells you because you have found that he is capable of deception. Trust only what you can personally verify. It may sound cynical at first but think about it. Why would you believe that he has somehow suddenly become honest just because you found his SSL. He needs to earn your trust.

Affairs are fantasy and deception by definition. It does not mean your WH has a mental illness. In MindMonkey's example, the guy was diagnosed and prescribed medication that the guy stopped taking.

Has your WH been diagnosed with mental illness? Have you ever suspected that he was mentally ill before this?

Also I know it's your SIL and not sister, but this is a good thread.

Also, listen to the clips at the end of the thread.
Husband's Affair with My Sister

CC, have you snooped and gone through all of your husbands emails, call logs and texts to verify this story? I does seem like there is a huge missing gap in this story.

I know you think you know your SIL but we know cheaters. And cheaters always lie. We have heard the most amazing stories on this forum and they turned out to be lies. This is why it is so important to do some snooping.

It is important for you to sit back and remove any narrative given to you by your H and SIL and just look at their actions alone. Look at the actions coupled with the evidence you have and what story does it tell you?

Please remember that you don't know for a fact that it was all taking place in his mind. All you know is that WH and SIL want you to believe it was all taking place in his mind.

I think something happened 5 years ago, and that is the missing piece of this puzzle. You need to find out what happened that caused him to fall in love with her at that time. She must have been making huge lovebank deposits, and that couldn't have happened if it was "in his mind only" and there was no contact between them.

She may have "basically" ignored it, but she didn't refuse all of his calls and ignore all of attempts to contact her. For all you know, she could have been contacting him as well. Moreover, she seems to be blaming her pastor for her failure to tell you the truth. Can you discuss this situation with her pastor? I wouldn't be surprised if she actually went to her pastor because she was feeling guilty about her inappropriate relationship with your husband, and seeking advice on how to end it.

Based on all of his phone calls, letters, etc. to SIL, he has no problem communicating or expressing his feelings. He has simply been withholding information from you. That is not a communication problem; it is an honesty problem.

Also, if I understand correctly, your SIL lives 4 hours away, and she and your brother got divorced 20 years ago. (Please correct me if I got those facts wrong.) I'm wondering why a former SIL who lives that far away would still be in close contact with you (and your husband) 20 years after she divorced your brother.