How is the estate of the father (a house in the village and a sum of money) to be distributed, taking into account that the father bought an apartment for his son and registered it in his name before his death, and his will to the son was to look after his sisters (daughters) when selling the house in the village?

A father must be fair among his children in giving gifts. It is not permissible to single out one of them with a gift without a legitimate reason. If a father singles out one of his children with a gift and then dies, there is a difference of opinion among scholars regarding the validity of the gift. The majority opinion is that it is valid. However, some, like Ibn Taymiyyah, hold that an unjust gift must be returned to the inheritance.

Therefore, the gifted apartment must be returned to the estate to be divided among all heirs, unless the sisters willingly consent to you being singled out with it, in which case the singling out becomes permissible.