1. Marriage with one's children, grand-children, great grand-children, etc. is not permissible. Nor is marriage with one's parents, grand-parents, maternal grand-parents, etc. permissible.
2. Marriage with one's brothers, uncles and nephews is not permissible. According to the Shari'ah, a brother is one whose mother and father is the same, or they have one father but two mothers, or one mother but two fathers. They are all brothers. But if the father is different, and the mother is also different; that person will not be a brother. Nikah with him will be valid.
3. Marriage with one's son-in-law is not permissible. This is irrespective of whether the daughter is already living with him or not. In all cases, nikah with him is haram.
4. A girl's father passed away. Her mother married another person. However, before the mother could even live with her new husband, she passed away or he divorced her. In such a case, the girl can marry this step-father of hers. However, if the mother lived with him, it will not be permissible for this girl to marry him.
5. Nikah with one's step-children is not valid. In other words, if a man has several wives, then one of the wives cannot marry the children of the co-wives. This is irrespective of whether she had lived with her husband or not. Nikah with these children is prohibited under all circumstances.
6. It is not permissible for a woman to marry her father-in-law or even the father or grand-father of her father-in-law.
7. 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.
8. 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.
9. A man married a woman. As long as he remains married to her, he cannot marry her maternal and paternal aunts and nieces.
10. 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.
11. If a woman and her step-daughter marry a person at the same time, the nikah will be valid.
12. Adoption is not considered in the Shar i 'ah. By adopting a boy, he does not become one's son. It is therefore permissible to marry one's adopted son.
13. 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.
14. Two women who are not blood sisters but are maternal or paternal cousins are permitted to marry one man at the same time. In the presence of such a cousin, another cousin can also marry the same man. The same rule applies to a very distant maternal or paternal aunt. That is, the niece and this distant maternal or paternal aunt can marry the same man at one time.
15. All the relations, which become haram on account of lineage also become haram on account of breast-feeding. In other words, if a girl is breast-fed by a particular woman, then this girl cannot marry the latter's husband because he will now be regarded as her father. A girl who has been breast-fed by a particular woman cannot marry a boy who has been breast-fed by the same woman. Nor can this girl marry the children of this woman because she is also regarded as a child of this woman. All the maternal and paternal uncles and maternal and paternal nephews who become related due to this breast-feeding also become haram on this girl.
16. If two girls have been breast-fed by one woman, they cannot marry the same man at one time. In other words, whatever has been explained previously, will also apply to relations based on breast-feeding.
17. A man committed adultery with a certain woman. Now it will not be permissible for her mother or her children to marry this man.
18. Due to the passions of youth, a woman touched a man with evil intentions. It will now not be permissible for her mother or her children to marry this man. Similarly, if a man touches a woman with evil intentions, her mother and her children will be haram on him.
19. In the middle of the night, a man decided to awaken his wife. However, he mistakenly touched his daughter or his mother-in-law. Thinking them to be his wife, he touched them with the passions of youth. Now, this man will become haram on his wife forever. There is no way in which she can become permissible for him. It will be necessary for him to divorce his wife.
20. If a boy touches his step-mother with an evil intention, she will become haram on her husband. There is no way in which she can be halal for him. If the step-mother touches her step-son with an evil intention, the same rule will apply.
21. A Muslim woman cannot marry a man who belongs to any other religion. She can only marry a Muslim man.
22. A woman's husband divorced her or he passed away. As long as she does not complete her iddah, she cannot marry anyone else.
23. Once a woman marries a man, she cannot marry another person unless and until she is divorced by this person and also completes her iddah.
24. If a woman is not married and she falls pregnant due to adultery, it will be permissible to marry her. However, it will not be permissible to have intercourse with her until she delivers the child. But if the woman marries the same person who had committed adultery with her, it will be permissible for the person to have intercourse with her.
25. If a person has four wives, he cannot marry a fifth woman. If he happens to divorce one of his four wives, another woman cannot marry him until the one who is divorced completes her iddah.
26. The marriage of a Sunni girl with a Shi'ah man is not permissible according to the majority of the ulama.( By Moulana Ashraf Ali Thanvi RH).