Is the sale by a father of agricultural lands to his youngest son, with the approval of the father, the son, and two sisters, and without the knowledge of the eldest sister who previously benefited from the father's money, considered an injustice to the sisters, and what is the ruling on the two sisters retracting their approval?

If your father's sale of the lands to you was a genuine sale, without any favoritism (muhabah), then it is permissible, and the permission of the other children is not required. However, if it was a fictitious sale or involved favoritism, then it is a form of gift (hibah) in which justice among the children is obligatory. If your sisters willingly agree to you being exclusively designated for the gift, then that is permissible, and they cannot retract their permission afterwards. The stable circumstances of the eldest sister do not justify singling you out for the gift over her. Adjusting the gift should be according to the inheritance shares, unless the sister has previously received a gift, in which case it is permissible to designate you in a way that achieves justice between you both. Some scholars are of the opinion that it is permissible to favor a dutiful son, or one who is in need, or one who is obedient.