What is the ruling on discriminating between children in treatment and gifts, such as favoring one of them in preparing her for marriage, or buying gifts or real estate for one of them without the others? And is the aggrieved party entitled to demand this from her parents?

Islamic law commands justice among children and forbids preferring some over others. The preponderant opinion is that justice among them is obligatory in gifts, preparing daughters for marriage, and purchasing homes. So, whoever gives something to one of their children must give the like of it to the rest. As for preferring some children due to a need, there is no harm in it. However, what is done without justification is impermissible, and the obligation is to equalize by returning what some were preferred with, or by completing the share of the rest. Designating an apartment for a son without justification is impermissible, and the one who is preferred should not accept an unjustified gift. If the eldest sister has been wronged, she has the right to confront her parents and demand justice from them, advising them gently, or to seek assistance from someone who can advise them, bearing in mind that the right of parents is great and honoring them does not cease, regardless of their condition.