Is it permissible for a father to bequeath a portion of his estate to one of his children after his death in exchange for that child having financed his brother's marriage during the father's lifetime? And if it is not permissible, how can the family atone for the father after his death?

If a father bequeaths a preference to one of his children, the will does not become effective unless approved by the adult, mature heirs. To rectify the father's error, the will must be rejected, and every rightful person must be given what is due to them. It is permissible for a father to spend on the marriage of his needy son, and he is not obligated to bequeath the same to his other unmarried children. Quoting Ibn 'Uthaymeen, it is wrong for a father to bequeath to his younger children an amount equivalent to what he spent on the marriage of the older ones; this will is null and void and forbidden, and it is not to be enforced after his death.