As long as a sister is married to her husband, it is not permissible for another sister to marry this brother-in-law of hers. However, if her sister passes away or he divorces her and she completes her iddah, it will be permissible for the other sister to marry her brother-in-law. In the case where the brother-in-law divorces the first sister, it is not permissible for the second sister to marry her brother-in-law until her sister completes her iddah.
If two sisters marry one person, the marriage of the sister whose nikah was performed first will be valid while the marriage of the sister whose nikah was performed later will not be valid.
A man married a woman. As long as he remains married to her, he cannot marry her maternal and paternal aunts and nieces.
If the relationship between two women is such that if we had to regard one of them as a man, their nikah will not be valid, then such two women cannot marry a person at the same time. When one of them passes away or one of them is divorced and completes her iddah, only then will it be permissible for the person to marry the other woman.
If a woman and her step-daughter marry a person at the same time, the nikah will be valid.
Adoption is not considered in the Shari’ah. By adopting a boy, he does not become one’s son. It is therefore permissible to marry one’s adopted son.
If a man is not one’s real uncle but he becomes an uncle through some other distant relationship, marrying him is permissible. Similarly, if a man happens to be one’s paternal uncle or nephew through some distant relationship, nikah with him is permissible. Nikah with one’s cousins is also valid irrespective of whether they are paternal or maternal cousins.