Incestuous relations are totally against the natural instinct of human beings. The relationship between close relatives should be based on respect, love, and kindness. Incestuous marriage is instinctively abhorrent to people of sound nature and is detrimental to the fabric of the family, which is the basis of society.
The eminent Muslim scholar Sheikh Yusuf Al-Qaradawi wrote about this matter in his well-known book, The Lawful and the Prohibited in Islam:
It is permanently haram for a Muslim man to marry a woman who belongs to one of the following categories:
1. The father’s wife, whether divorced or widowed. During the period of jahiliyyah (pre-Islamic era of Ignorance) such marriages were allowed. Then Islam prohibited them, for once a woman is married to a man’s father she acquires the status of his mother, and this prohibition is out of honor and respect for the father. Moreover, as this inviolable prohibition leaves no room for sexual attraction between the son and his step-mother, they are able to develop a relationship of respect and honor.
2. The mother, including the grandmothers on both sides.
3. The daughter, including the granddaughters from the son or daughter.
4. The sister, including the half- and step-sisters.
5. The paternal aunt, whether she is the real, half-, or step-sister of the father.
6. The maternal aunt, whether she is the real, half-, or step-sister of the father.
7. The brother’s daughter, i.e., his niece.
8. The sister’s daughter, i.e., his niece.
All these female blood-relatives are a man’s muharramat and he is mahram to his corresponding female relatives. Marriage to any mahram whomsoever is permanently prohibited. The reasons for this prohibition are as follows.
a. Going into any sexual thoughts concerning such close relatives as one’s mother, sister, and daughter is instinctively abhorrent to human nature; there are even certain animals which avoid mating with such closely-related animals. The respect a man feels for his aunts is like the respect he has for his mother, and likewise uncles are regarded as fathers.
b. Since the family must live together in intimacy and privacy but without incestuous relations, the sharia intends to cut at the roots of any sexual attraction among such close relatives.
c. Since there is natural love and affection among such close blood relatives, the intent of the sharia is to expand the circle of love and kinship by prohibiting incest and thereby directing the man’s search for women outside the family. Thus each marriage extends the sphere of love, bringing new people within this ever-expanding network of affection: [And He has put love and mercy between you] (Ar-Rum 30:21).
d. The natural sentiments of love and affection between a man and the above-mentioned female relatives must be kept strong forever. If marriage were permitted between such relatives, it would cause jealousies, dissensions, and the disruption of families, destroying the very sentiments of love and affection which give cohesiveness and permanence to the family structure.
e. The offspring of marriages to such close blood relatives would most probably be defective and weak. Moreover, if physical or mental defects are present in the members of a family, they would become more pronounced among the children of such marriages.
f. The woman needs someone to champion her rights and support her case against her husband, especially when relations between the two of them become strained. If those women who could defend her became rivals, how would this be possible?