First of all, the mother should have been distributed the property equally to his children; since it is mustahab (desirable) to give equal shares to each child while giving gift to them. It is sin to discriminate between them with no valid reason or giving to some and depriving the other. However, if she did so and gifted the property in his possession, the transfer is complete and child is owner of the property.
But, if she did not separate the gifted portion and did not give it in his possession, rather she did only oral gifting, it will not be valid as per the Shariah. In this case, all the heirs will have their due shares in the property after the death of mother.
Allah (Subhana Wa Ta’ala) knows Best
Darul Uloom Deoband