It is allowed in Islam to marry 4 at a time. Why it is prohibited to marry sister of wife when the wife is alive?. Please provide me with detailed answer

1. It is permissible for a man to have four wives at once on condition the man can maintain equality among all his wives, the rules of which are very strict. If a man cannot maintain equality, then it is not permissible for him to enter into a polygamous marriage. Allah states, ?If you feel that you will not be just, then marry only one.? If a person in a polygamous marriage does not maintain equality among his wives, he will be severely punished by Allah on the day of Qiyaamat.%0D%0A%0D%0A2. According to the Shari?ah, it is prohibited to have two sisters in one Nikah. This is mentioned in a hadith. The logic behind this is the relationship behind the husband and wife is a special one. It is generally observed that both are possessive over one another. Such a relationship with two sisters in one marriage will lead to acrimony between the sisters and break their relationship, whereas the Shari?ah has emphasised on maintaining family ties.%0D%0A%0D%0Aand Allah Ta'ala Knows Best%0D%0A%0D%0AMufti Ebrahim Desai