Hi B.,
It is important to understand that in a long-term marriage (15 years in your sister’s case) we no longer think in the term of “I”. Our partner is a part of us. When our spouse leaves, a part of us is now missing. The emotional gamut of feelings suffered is not unlike if the person who left has died. The problem is that there is no real finality of ceremony to help with the grieving process.
When the spouse who left begins to see someone else it only adds to the feelings of abandonment, loss, and betrayal. If the person they are seeing is someone we know it amplifies those feelings. We feel the friend or acquaintance should understand what we are going through and not be willing to add to our grief. It is a set back to the healing process, making it difficult to move on or to think in terms of a solution.
Your sister needs you right now. She is still in the healing stage and now been wounded even more deeply. With a year long separation she has had the carrot of hope hung in front of her that her marriage might be saved. She is going to need the help of your strong support as she learns to be strong again. It is possible to be supportive without taking sides. When she cries put her head on your shoulder and hold her. When she vents, simply listen and let her get out all that is welled up inside. Try to remind her of her personal strengths, talents, and gifts. Always reassure her she is loved.
As long as the focus is on the husband and the former sister-in-law relationship the energy needed for remedy or healing is weakened.
My prayers are with you and your sister,
L.