Yes, a son can marry his step-sister in Islam, provided there is no blood relation (consanguinity) or relation through breastfeeding (fosterage) between them.
Islamically, the prohibition of marriage is based on specific relationships defined in the Quran and Sunnah. These include:
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Consanguinity (Blood Relation): Marriage is prohibited with close blood relatives such as mothers, daughters, sisters, aunts, and nieces.
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Affinity (Marriage Relation): Marriage is prohibited with certain relatives by marriage, such as a step-mother or a daughter-in-law.
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Fosterage (Breastfeeding Relation): If a child is breastfed by a woman other than their mother, specific relations are established, and marriage may be prohibited with certain members of the foster family, similar to blood relations.
A step-sister, by definition, is the daughter of one's step-parent or the step-parent of one's parent. There is no blood relation. Unless the step-sister has become a relative through breastfeeding (meaning the son was breastfed by the mother of the step-sister or the step-sister was breastfed by the mother of the son), marriage is permissible.
In Summary:
| Relationship Type | Marriage Permissible? |
|---|---|
| Blood Relation | Generally No |
| Affinity | Sometimes No |
| Fosterage | No (similar to blood) |
| Step-Sister (no other relation) | Yes |
Since a step-sister is not a blood relative or a relative through fosterage (in most cases), there is no Islamic prohibition against marriage. Therefore, a son can marry his step-sister in Islam if no other prohibited relationship exists.
