What is the legal ruling in Islamic law for a man who commits (incomplete) adultery with his wife's sister, and is the wife obligated to divorce him?

Adultery with the wife's sister is a grave abomination. There are no authentic hadiths that prove a curse upon one who combines his fluid in the womb of two sisters. Adultery with the wife's sister does not prohibit the wife, even if it is said that it is better to avoid cohabiting with her during the period of purifying her sister's womb from the fluid of adultery.