Is it obligatory to abide by a prior agreement among the children regarding the division of inheritance, especially when there is implicit objection from one of the heirs? And what about the disposal of apartments that the children inhabited and invested in before the death of the parents?

If a mother gifts two apartments to two of her sons, excluding their other siblings, without a legitimate شرعي justification, then this is impermissible, and she must revoke the gift or give an equivalent amount to the remaining siblings. If the mother dies before revoking the gift, and it had already been taken possession of during her lifetime, most scholars hold that the gift becomes effective upon death before it can be amended. Nevertheless, the sons should amend the gift to rectify the injustice towards their siblings. If the mother revokes the gift during her lifetime, the two apartments become part of the inheritance and are to be divided among the heirs. As for paying an amount for furnishing the apartment, if it was a donation, it is considered part of the inheritance; if it was not a donation, the person who paid the amount has the right to reclaim it from the heirs.