Is the son whose father wrote all his property to him, thereby disinheriting the remaining siblings, sinful, or is the sin only upon the father?

If a father designates all his properties to one of his children by way of a will, it is a will to an heir (وصية لوارث), which is not enforceable unless the other heirs consent. If it was a gift during his lifetime, he must be fair among his children; otherwise, the gift is void and must be returned to the estate after his death.

If the father did this with the intention of disinheriting other heirs, he is disobedient (عاصٍ) and has committed the major sin of injustice in his will (الجور في الوصية). The child must return the properties so that they can be distributed according to Islamic law; otherwise, he too is disobedient.