Well, first of all, it would most likely lead to the end of the marriage. The wife would probably feel extremely violated and humiliated. The sister would be in a very wrong and unethical position. Emotionally, all parties would be in turmoil. There could be legal issues in some places as well, especially if there are accusations of coercion or abuse. Socially, they would be shunned by their family and friends. And from a psychological perspective, it could cause long - lasting damage to their self - esteem, mental health, and ability to form healthy relationships in the future.
One major consequence would be the breakdown of the marriage. The wife may feel betrayed and violated, and the sister - if she were to go along with it - would also be in a very difficult and immoral situation. Socially, they could face ostracism as this behavior goes against social norms. There could be long - term psychological effects on everyone, such as guilt, shame, and depression.