If you've found out that your sister is cheating on her husband, life has just offered you the grand opportunity of shining a light into a dark place into her life. If you handle the situation right, you'll be able to offer your sister a loving but honest ear while encouraging her to make the choices that will most benefit her family and herself.

Let your sister know that you're concerned about her decision to have an affair. Tell her that her affair is the wrong solution to what may be a bad marriage, advises Jack Marshall, President of ProEthics, an ethics-training company. Be kind and non-judgmental when having the discussion, striving to have compassion for the circumstances that led to the affair. Nevertheless, feel free to let her know your full concerns about how the affair can harm her, her family and the other person involved.

While it's appropriate for you to address your concerns with your sister, it is out of bounds to do so with her husband. While Psychology Today columnist Mark D. White says it is in the best interests of a marriage for the other partner to be aware of an affair, the decision lies in your sister's hands, not your own. For all you know, she may be planning to leave the marriage and the disclosure of an affair would merely add to the pain of a failed relationship. Remember that you have no way of knowing what goes on behind closed doors.

No doubt you'll find Thanksgiving dinner difficult as your possibly oblivious brother-in-law sits across from you making conversation. Don't make family get-togethers even more awkward, however, by blabbing to your mother or any other relative. One reason why an affair is so damaging to a marriage is because it brings another person into the sanctity of the relationship. If you have loose lips, you'll bring in several more people and their opinions into a situation that should be dealt with privately between two people.

Encourage your sister to seek support, as she is apparently having difficulty navigating this portion of her life. She may want to seek individual counseling to help her sort through her priorities and to address feelings that may need to be out in the open. Her marriage might benefit from marriage counseling. Share any good contacts you know without being overbearing. Offer to lend an ear yourself, as listening to her concerns about how to cope with this poor decision is not the same as condoning her behavior.