Is it considered unjust to the older siblings if a father allocates to his youngest son a shop and an apartment whose value is significantly greater than what he allocated to his older siblings, after real estate prices have risen, and how can the father convince them that there is no injustice in his action?

There is no objection to a father spending on his adult children for their marriage if they are in need. He is not obligated to give the younger ones the same as them, but he must marry them off when they grow up and are in need. As for a father gifting an apartment to one of his children, this is not considered a form of maintenance (nafaqah). Rather, it is a gift that requires justice among the children, so he must give the other children similar apartments to those given to their siblings.