I beg to differ from Mr. Bharat Chugh.
B's title is from Y through her GPA holder A.
Even if there was no GPA and no sale to B had taken place, the original title for Y is by way of succession upon the death of X, her father.
If, on the date of death of X, no children were born to Y, she was the sole successor. Then, in the date of registration of the deed in favour of B, if no children had been born to Y, the title flow to B would be clear. A's GPA did not constitute any transfer of property. Only the exercise of power conferred upon A by the GPA executed by Y, concludes the transfer of property, that is on the date of registration.
The the children, who are stated to be minors, were born on or before the date of registration of the deed in favour of B, then title flow is not clear. For Y1 and Y2, the property is ancestral and they have a share by birthright. However, the 1/3 share of Y could be transferred by her.
So, the intending purchaser has to be cautious. Hundreds of suits for partition are filed and pending covering exactly the similar circumstances. Minors upon attaining majority and in some cases after several years of attaining majority claiming that the alienation of the property has come to their knowledge, only recently, to create limitation are being filed day in and day out. The present title holders who have been bona fide purchasers for valuable consideration, are at a disadvantage to challenge the myriad complicated relationships pleaded. Often these are collusive and induced cases. Since property prices have gone sky high, past vendors turn greedy and attempt harassing present owners by filing such suits and dragging them for several years, to pressurise them to part with more money to purchase peace by getting cases settled by compromise.
In this case, though Y has transferred the property and enjoyed the money, there is no guarantee that Y1 and Y2 , on their own, or being induced by others, will not file cases to undo the transfers effected till they'came to the knowledge of the sale(s)', which by law is upto their completing 21 years but stretched far by the ingenuity of lawyers.
In my opinion, the property is not worthy of consideration for purchase.
This opinion is based on facts given and may get modified, if more facts are presented. Also, the above is based on Hindu law of succession only.