What is the ruling of the Sharia on a father's failure to be just among his children in giving gifts, and is he sinful for that, and what is the ruling on the money he gave to one child over another?

If the value of the two plots of land is equal or close, and there is a significant disparity in what the father helped his two sons with, then the reason for this preference should be examined. If it was for a legitimate Shar'i reason, such as one of them having a special need, illness, large family, or similar circumstances, then there is no blame on the father. Otherwise, it is not permissible according to the preponderant opinion of scholars, and the excess amount of the gift must be returned, or the one who was not preferred must be given what achieves justice.