Eight hundred years ago, a sect called “Batinyeh” or “Qaramata” emerged in the Islamic world. One of the sect’s notable leaders, Obaidullah bin Hassan Qairwani, wrote a fascinating letter to his followers, in which he guided them on the way of life.
In this letter, he writes: “I don’t understand this foolish notion that within families there exists a beautiful and tasteful girl named sister. He knows his brother’s character and is familiar with his temperament; yet this foolish man gives her hand to a stranger, not knowing whether his sister will be happy with that stranger or not. Does she know his character? At times, this same brother brings home a girl who is not equal to his sister in terms of beauty, taste, and temperament. What is the reason for this foolishness? Why give the property of our household to someone else and receive something in return that does not guarantee our peace and comfort? This is nonsense and against reason! I advise my followers to avoid this folly and keep the family property within the family.”
In another part of his letter, Obaidullah bin Hassan Qairwani presents this argument based on wisdom: “What is the reason that when a sister can cook food for her brother, alleviate his hunger, prepare his clothes for his comfort, and make his bed, she cannot satisfy him sexually? What is the reasoning behind this? It is against reason.”
You may criticize this theory as much as you wish, but I assert that if one attempts to respond to this argument purely from a rational perspective, devoid of divine revelation, it cannot be adequately answered based on reason alone.
If someone claims that this perspective is morally reprehensible and burdensome, the response is that the terms “immoral” and “ugly” are constructs of our surrounding society. You were born into an environment where this matter is considered a defect, which is why you perceive it as such; otherwise, it is not a flaw in an intellectual sense. If you argue that lineage will be compromised by this marriage, then what is inherently wrong with that? Which moral principle necessitates preservation?