A Muslim is prohibited to have two wives at a time, if these two wives are related to each other by

A Muslim may not have at the same time two wives who are so related to each other by consanguinity, affinity or fosterage that they could not have lawfully married each other if one of them had been a male, for example two sisters, or an aunt and her niece. This bar is called unlawful conjunction, and in Sunni law a marriage in breach of it is irregular. Since the bar covers all three kinds of relationship, each listed relationship is a ground.