Nominee is only trustee and not owner.  Ownership rights are through succession laws. So nominee can't sell the house if he is not a owner as per sucession law

1) nominee is only trustee for legal heirs 

2) on demise of father in law his wife , 3 sons and one daughter have equal share in said flat 

3) during his lifetime father in law can execute gift deed for transfer of flat  if he so desires 

If sale deed is in name of nominee by housing board she would be absolute owner of the property 

if she has sold the flat then original flat owner has no share in property 

Nominee is only entitled to do paper works on behalf all the legal heirs. Nomination can not substitute law of inheritance of intestate succession.

So if he makes his wife sole beneficiary of Will or make a gift deed in her favour then only she alone can transfer the flat. Else all the son and daughters need to be made party in the sale deed.

Get all your papers checked  y an advocate before proceeding ahead.

After the death of original owner of the flat the legal hairs will be the ultimate owner  the flat and increase the flat has been transferred in anyone's name because of others relinquish  their rights in favour of one person after the death of the owner of the property her legal hairs will have the share in the property

Dear Sir,

Rights of the nominee. ... The nominee shall hold the flat/unit in trust until all the heirs are brought on record and shall not have the right to ownership and shall not create any third party interest. The society is not concerned with disputes amongst the heirs of the deceased

The word nominee means a person who holds or acquire right, property or any other kind of liability incurred on behalf of others. Nominee means a trustee. A nominee holds a property on behalf of other legal heirs.

Whether it is advisable to make a nominee in case of joint ownership of flats? The object behind nomination is to avoid confusion in case there are disputes between the heirs and legal representatives and to obviate the necessity of obtaining legal representation and to avoid uncertainties as to with whom the society should deal to get proper discharge. Nomination does not create a new rule of succession. Therefore, it is highly advisable to make a nomination in case of joint ownership of a flat. In case of a simultaneous death of both the joint owners, the flat is rendered intestate. An appointed nominee and a will (of a joint owner bequeathing his/her ownership in the name of the other joint owner) can go a long way in avoiding unnecessary confusion and ambiguities. In a case where both nomination and Will is prepared, the Will will prevail over the nomination paper.

Nominee has no right. How flat trasfer to wife? Sale/gift deed ?  If yes than sale by wife is valid. And after buyer death, flat will inherit in his legal heirs equally.

A nominee can not sell the flat. The legal heirs should get Letters of administration from Court if they want to sell a Flat. 

The nominee cannot sell the proeprty especially when the owner is alive.

The sale executed by the nominee can be cancelled by filing a suit for cancellation by the legal heirs of the deceased flat owner.

There has been a fraud taken place in this regard.

The nominee cheated the blind man who is the owner of the flat.

Hence she is  liable to be punished for this fraudulent act  besides cancellation of the registered sale deed executed by her in the capacity of nominee.

Dear

The nominee can not become the owner of the flat and if housing board have transferred the flat on name of wife then the procedure for transfer must be followed completely. That is legal heir certificate and consent from both daughters must have be taken before transferring the flat on name of mother. 

And if transfer done by board then lady become absolute owner of flat and sale done by her is valid.

And buyer will become absolute owner after purchase if flat and after his death his children are entitled to get equal shares from his flat.