OK, I will tell you my story. One of my relatives came to me because he had found his soul mate. Unfortunately, he was already married to someone else, someone he felt did not understand him at all. His soul mate, meanwhile, needed his emotional support.
This is what I told him. A) Your soul mate may indeed need emotional support, but what she does not need is emotional support wrapped up in the package of an emotional affair with a married man. That she needs support does not mean you are a good choice to supply what she needs. More to the point, B) he needed to do the work on his first commitment and figuring out what was wrong with it before he jumped into another commitment. I have just known too many people who either realized that their first marriage had not been given a fair chance or who jumped into a second marriage with all the same problems that the first one had, a truth obscured by the thrill of a new romance until it was too late.
So, OK, he breaks it off with the friend to whom he was becoming inordinately attached. He started going to counselling with his wife, to find out why their marriage wasn’t working and to make an intelligent decision about whether to keep living together without the muddying influence of someone standing behind Door Number Two.
He is still married to his first wife, and they are very happy. By doing so, he has also increased the chance that his children will remain married when it is their turn. He learned things about himself that improved his other relationships, as well. He also avoided the expense and heartbreak of going through a divorce, and even those who have gone through the most “amicable” of divorces will tell you that this is well worth avoiding like you’d avoid setting your own hair on fire. If you are not protecting your sister from the chance of wrecking her marriage and going through a divorce, you are not in her corner. You are enabling her in her self-delusion. My brothers wouldn’t do that to me, and thank Heavens for them! They can keep their feet on the ground for me when I’m being stupid. That is real compassion, because it is caring that cares even when it isn’t liked for caring. Your parents aren’t compassionate when they are more interested in being friends than being parents. The same can be said of anyone we ought to be able to trust to hold our feet to the fire and choose to be the best version of ourselves. That, Bro, means you.
Meanwhile, I think I mentioned the other people I know who have regretted their decision to jump out of a marriage that they haven’t tried to fix. Others have disgusted their children, friends and relatives by “turning” on a spouse suddenly and leaving without warning and without any real opportunity for rehabilitating their marriage. Yes, these were the people who had witnessed the vows and knew what a different song he was singing than on his wedding day. They do not tend to like the “new person,” whom they see as a self-centered home wrecker with no conscience.
Tell your sister to work her way though her marriage problems, and to satisfy herself that she had exhausted her options for fulfilling her vows. If it is impossible to live in peace, she may separate with the bond remaining. Until then, she should not have any social life that she doesn’t disclose fully to her spouse and she should avoid situations with her friends that present a near occasion of the sin of more serious physical infidelity. Physical adultery is a pit that can be fallen into, and many good people have allowed themselves to fall into it because they were arrogant about their ability to be “just a little bit unfaithful.” She should not allow that to be her. If she does her work and in the end finds honestly that the marriage was invalid, she can be declared free to marry. She’ll know the mistakes she made in the first marriage, and if she marries again she will be far more likely to have a valid attempt instead of an invalid one. Oh, and think of how happy she will be, if she finds she can grow her current and true marriage into the best version that it always could have been, and yet never had been! (I have seen this happen, too…

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I myself had a very close friend who could have become my soul mate. He does understand me in ways my husband never will. To have left my faithful husband for him would have been a gross betrayal of my husband but also a disservice to myself. Tell her to stop and turn back, before it is too late.
Go the way of integrity. Live a life that can be lived out in the open. It is worth the trouble.
Give her the support of recognizing how hard this is. Give her the support of doing more listening than talking. Do not, however, allow her to sell you her self-delusion and call that support. That is BS. (
bad
support…)