How is the inheritance legally divided among siblings given that the father has made a gift to his children? Is the value of the land and apartment calculated at the time of sale or at the current time of division? And is the father sinful for giving a gift to one of his children to the exclusion of others?

It is not permissible for a father to favor some children over others in gifts, unless there is a legitimate reason or their consent. If the siblings previously agreed to allocate the apartment to you, then there is no blame upon you for accepting it, and you are not obliged to include them in its price. However, if they did not agree, then justice must be observed in dividing the apartment and including all siblings, male and female. When it is sold, its price should be divided equally among them, and the share of the deceased sister should be given to her heirs. As for the value of the land, what is considered is the price at which it was sold, and your share of that price remains an obligation upon your brothers.