The property of father in law was acquired by his wife, hence she becomes the absolute owner of the property, therefore the subsequent transfer by her to her daughter in law of the property by executing a gift deed, is very much valid and by this transfer the daughter in law becomes the absolute owner of the property.
Hence she can very well sell the property or alienate the same in any manner she may feel so or desire or decide.
Nobody can question her authority over this property which absolutely belongs to her.
Since this is not ancestral property and the property was acquired by the daughter in law by a registered gift deed, the daughter in law acquires clear and marketable title to the property hence she need not obtain permission or consent from her children or anyone to sell this property.