Will the father be punished for his action regarding this inheritance, in which he designated a portion of his property for his twin sons?

The majority of scholars hold that equalizing gifts among children is commendable (mustahabb) but not obligatory. However, the Hanbalis and some other jurists maintain that it is obligatory to equalize among them, and that singling out some children or preferring them is sinful. The preponderant opinion is that a parent commits a sin if they favor some of their children without a legitimate শরعي (Shari'i) justification. In this case, Shaykh al-Islam Ibn Taymiyyah held that the gift must be returned to the estate after the death of the giver to be divided justly. If a Shari'i judge rules the gift valid, it settles the dispute, but it is preferable to return the gift if there was no legitimate Shari'i justification for it.