Does the apartment that the mother verbally promised her youngest son – with the intention of deeding it to him at the time of his marriage – count as an inheritance to be divided according to Islamic law after her death, or is it exclusively for the promised son?

It appears the son does not own the apartment because the conditions for a valid gift (hibah) were not met. One of the conditions for a binding gift is that the recipient (the donee) must take possession of the gift before the death of the giver (the donor). This did not happen. If either the donor or the donee dies before possession is taken, the gift becomes void. This is supported by the hadith of Aisha (may Allah be pleased with her), who narrated that Abu Bakr (may Allah be pleased with him) gifted her twenty measures of dates from his property in Al-Aliyah. When he fell ill, he said: "O my daughter, I had gifted you twenty measures of dates. If you had taken possession of it, it would have been yours. But today it is inherited property, so divide it according to the Book of Allah."