Since the Flat is in the name of your husband and his brother, it is to be seen how the purchase amount for the Flat had been paid, i.e. by your husband only or his brother had also contributed. If your husband had paid the entire amount then the Flat belongs to your husband and his brother's name was only for convenience. This has to be seen from the Income Tax Returns of your husband if the Flat is shown entirely in his name.
If a nomination is filed, the Society is bound to transfer the Flat in the name of the Nominee who can deal with the Flat until such time the other legal heirs make a claim or file proceedings to establish their shares.
The brother cannot sell his share if he has not contributed any amount towards the purchase of the Flat and if he has contributed any amount towards the purchase of the Flat he can file a suit for partition or sale of the Flat and distribute the proceeds amongst the legal heirs of the deceased (i.e. your husband) and himself.