How to make release deed

The 1/2 share of Mr. B in the flat shall devolve upon all of his legal heirs in succession as per personal laws of the deceased.    

Nominee is just a care taker of the property till it is handed over to the legal representative(s) of deceased.  Nominee has neither any interest or claim in the property left behind by Mr. B (since deceased).

Stamp duty and registration fees differs from State to State, which will be applicable where the property is situated.

Formats of RD is not provided on this platform, you will have search it.

Flat have first owner Mrs A & Second owner Mr B having two sons also mentioned nominee , Mr B was no more

Mrs. A owns 50% share

Mr. B owns 50% share, in absence of Mr. B his 50% share will devolve equally amongst Mrs A and the two sons. 

The release deed is to be prepared according to the above divisions. 

Stamp and registration charges differ Statewise

If B's  children are desirous of relinquishing their rights in B's share of property and if B is reported to have died intestate, then they both can execute a registered release deed in favor of A, the applicable stamp duty and registration charges as applicable in your state have to be paid.