in your question you not mentioned that whether that flat was his self acquired property or not ?
let us presume it was his self acquired property and answer is behind your brother following persons are legal hearers:- son, wife, father and mother.
therefore the best option is transfer the ownership on all of these person in-spite of only son or only wife.
For the purpose of son make his mother as legal guardian and his mother and father will may be co-sharer.
to transfer ownership you have to apply to city survey office and apply for entering there name along with death certificate, Id proof of applicants, birth certificate of minor son, affidavit for legal hearers, and application duly stamped. same application you have to file in society office also to enter name in share certificate.
If wife i.e. widow claim ownership you can take objection to enter other sharers name on record once your objection is recorded no any officer will enter only her name on record.