بسم الله الرحمن الرحيم
If Umar gave all his property to the said son of his sister-in-law and made him its full owner and withdrew his rights from the property then as per the Shariah his sister-in-law’s son shall be the owner of the gifted property. As for the properties which have been discovered after the death of Umar, if Umar’s ownership is proved in those properties then they shall be divided among his legal heirs as per the Shariah.
We can only write the share of each heir after we receive the detail of Umar’s Shariah heirs after his death till now.
Allah knows Best!
Darul Uloom Deoband