Uslub Bayan (Al-Nesa Ayah25-26)
First, an adjective indicates determination. example, the one who lies just occasionally is not necessarily a liar. A man to be called a liar must be a habitual one. Zani and Sariq (thief) indicate the habitual ones. Therefore, the word ‘Mother’ indicates the determination of a woman, who feeds a child as a foster mother. Second, in verse 4: 43, the Quran mentions sick and travelers being exempted from keeping fast. But the cause, that is, undue difficulty, can extend the exemption to others who suffer from the undue difficulty. So, the mention of the foster sister extends the cause of foster relation to be forbidden for marriage to other women who are the blood relations to the foster mother.