Beth — I agree. When I started writing about this I felt so alone. Now, people are coming out of the woodwork on both sides and I see how common it really is, sadly.
I hope that your sister-in-law will be able to find some of the peace and acceptance that I have found. It has taken so much work, but I know my life is better because I was willing to let go. I have written several other pieces about my process through this heartbreaking experience. I hope that something I have written will help.
Feeling that you have to take sides must be so hard. I have never asked anyone in my family to do that, and it has been part of the process for me to learn to be ok with the fact that she is still in relationship with others. In fact, I am glad she is. She should not have to lose her entire family because of a choice she made about me.
My heart goes out to your family. Know that you are not alone. ♥