Is a wife's telling her husband's sister about his illicit relationship with his female colleague considered backbiting?

If you are certain of your husband's involvement in haram (definition) acts, informing his sister for the purpose of consultation and seeking advice is not backbiting (gheeba) that is forbidden, provided you limit yourself to what is necessary and beneficial. It is advisable to work towards reforming your husband and helping him repent. However, if he does not repent and remains in his state, it is preferable to separate from him through divorce or khulu (divorce initiated by the wife), as this is recommended if the husband abandons a right belonging to Allah the Almighty.