It is permissible for a man, who

A: It is Haram to marry two sisters at the same time according to clear texts in the Qur'an and the Sunnah, whether they are full sisters, maternal or paternal stepsisters, biological or foster sisters, free women or bondwomen, or a free woman and a bondwoman. This was unanimously agreed upon by the scholars among the Sahabah (Companions of the Prophet Muhammad) (may Allah be pleased with them), Al-Tabi`un (Followers, the generation after the Companions of the Prophet) and the rest of the Salaf (righteous predecessors). Ibn Al-Mundhir said that it was unanimously agreed to, according to Allah's saying, Forbidden to you (for marriage) are: your mothers until His statement, ...and two sisters in wedlock at the same time, except for what has already passed The Prophet (peace be upon him) also stated, (Part No. 18; Page No. 235)  Whoever believes in Allah and the Day of Judgment should not marry two sisters at the same time. This is also proven by the Hadith related by Al-Bukhari and Muslim, on the authority of Um Habibah (may Allah be pleased with her) that she said, "O, Messenger of Allah! I would like you to marry my sister." He said, "Would you like that?" She said, "Yes, for I am not your only wife, and I like sharing the goodness with my sister." He said, "But she is not lawful for me." Thus, your sister's marriage to your husband while you are still married to him is Batil (null and void) according to the evidence cited. He should divorce her and perform Tawbah (repentance to Allah) from what he did. It is impermissible for your sister to allow him to have intercourse with her, as she is not considered his wife. He cannot have intercourse with you either, until your sister finishes her `Iddah (woman's prescribed waiting period after divorce or widowhood), which lasts for three monthly periods. If your husband does not offer Salah except in Ramadan, he is considered a Kafir (non-Muslim) according to the more authentic of the two opinions of scholars, even if he does not deny its obligation, according to the statement of the Prophet (peace be upon him), What makes one a Kafir and a Mushrik (polytheist) is abandoning Salah. Related by Muslim in his Sahih (authentic) Book of Hadith. (Part No. 18; Page No. 236) The Prophet (peace be upon him) also stated, That which differentiates us from the Kafirs is our offering of Salah. A person who abandons it becomes a Kafir. Related by Imam Ahmad and the Four Compilers of Hadith (Imams Abu Dawud, Al-Tirmidhi, Al-Nasa'i and Ibn Majah) with a Sahih Isnad (chain of narrators). There are many Hadiths that carry the same meaning. Accordingly, you should keep away from him and file for divorce from him. You should refer to the court to give you divorce papers. May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.