Taxability of Gift from step brother

Assessee is an individual AND received gift of Rs.  10 lakhs in A. Y.  2020-21. ASSESSEE has claimed it is not taxable as it is received from step brother.  However AO is not accepting contention of appellant on the ground that step brother or sister is not covered in the definition of Relative under Income tax Act and therefore he has made an addition in the hands of assessee with mentioning any section.  Is action of AO is correct and justified in Law.  Whether assessee has case to fight in an appeal.  Pl guide

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