Is it permissible for the children whose parents are different to remain alone in the same house?

My friend has a stepsibling (their mothers and fathers are different). One of them is eighteen and the other is nineteen years old. Their genders are different and they live in the same house. Is it religiously appropriate? What should they do?

Dear Brother / Sister,

The people whose mothers and fathers are different are non-mahram to each other; therefore, it is not permissible for them to remain alone in the same house.

It does not matter whether the spouse of the parent - even if it goes up – is related from the father or from the mother, whether they have had a sexual intercourse after marriage or not; the same decree is valid for the wife of the father or the wife of the grandfather (father’s father or mother’s father).

"And marry not women whom your fathers married,― except what is past: it was shameful and odious,― an abominable custom indeed." (an-Nisa, 4/22)

The verse above is evidence for it. What is meant by "nikah (marriage)" here is the marriage contract. It is a reason for being haram whether a sexual intercourse takes place or not. In Arabic, the word father is used for grandfathers and great-grandfathers too.

What is rendered haram by this verse is only the father’s wife. However, this woman’s daughter from another husband or her mother is not haram for her husband’s son from another woman. Therefore, it is permissible for a man to marry a woman and his son to marry the daughter or mother of this woman.

The reason why marriage through marriage kinship is haram is to show respect to ancestors. Beside, mischief is prevented and the improvement of families is realized. Besides, the mischief that could occur because of the son looking at his father’s wife and seeing her due to living in the same house is prevented. (Prof. Dr. Wahba Zuhayli, İslam Fıkhı Ansiklopedisi, IX/106)

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