First of fall grand daughter or grand son or daughter or son has no right in parents self acquired property.
On intestate demise the children and husband or wife are legal heirs not the grand children's
They cannot demand partition neither in self acquired property children can .
If after death of grandmother it is recorded in name of mother then the mother has complete right to sell transfer gift or make will of property.
Now if sale is fraudulent it can be challanged before court of law and it can be cancelled.
Now remedy for mother is that she can challange same before civil court for cancellation and can provide strict proof that fraud was done also a criminal case for same can filed against son and daughter in law.
The third party has no remedy If sale done mother as she has complete right.
Lastly if mother is senior citizen I.e. above 60 she can challange the said deed before the maintenance tribunal also though law for that has to be followed and complaint accordingly has to be drafted.