Please answer the following questions to study your case and offer proper adivce:

1. Are A's mother and his wife still alive? If not, when did they die - before A's lifetime or afterwards?

2. Is the property of A in question his self-acquired asset or his undivided share in his ancestral property?

3. In the partnership deed executed, did the sisters of B and C, namely D and E relinquish their respective shares in favour of B and C, having accepted cash compensation in lieu thereof?

A proper conclusion can be arrived at on B's title if only these queries are answered satisfactorily.

If it is self acquired property of deceased grand daughter consent is not necessary 

B can execute sale deed as partotion had been duly stamped and registered and daughters have signed the deed of partition 

It has to be drafted through a lawyer to rule out all possibility of error

Basically the partition of property X between B and C by excluding D and E is not legally valid and an illegal act.

Even though D and E obtained money in lieu of their rightful share in the property, the fact that they both have not relinquished their rights by executing a registered release deed remains an obstacle or a legal problem. 

The property X owned by A, assumed to be his self acquired property, shall not devolve beyond his next generation children to his third generation grandchildren since it does not qualify as an ancestral property hence the signatures of the grandchildren of A is not required while the B is selling his share in the property, bujt the problem i that the female siblings of B and C are required to execute a registered ratification deed to ratify the partition deed on the basis of receipt of money in lieu of their share in the property. The mutation of partitioned property between B and C will not confer absolute rights or title in their favor until D and E do not relinquish their rights in the property in the manner known to law. 

Therefore you may ask your vendor to obtain either a registered ratification deed or registered relinquishment deed from their female siblings before proceeding with the proposed purchase of property. and you can keep the proceedings pending until then. 

If it was ancestral property of A then gran daughters have share in ancestral property by birth 

consent of grand daughters is necessary for sale of property 

From your subsequent post it is clear that the female siblings of B and C have not executed a registered release deed, but just mentioned that they received money, which is not sufficient. 

They both should have explicitly executed a registered release deed to make the partition deed as well as their relinquishment legally valid, in the4 absence of the same, the title of your vendor B appears to be defective. 

If the property X was ancestral to A, what is the mode of acquisition of the property to A?

1. Ok, they are ruled out for any share.

2. An ancestral property is one which has been in common possession and enjoyment of all the co-sharers continuously for 4 generations and has not been divided by partition in between. Is this condition satisfied in A's case?

3. Are the sisters parties to the said partition deed and have they signed it?

Please clarify 2 and 3 for a proper advice.

-  Since the property was in name of A , then after his demise intestate , his property would be devolved upon all his legal heirs equally , including his wife if alive. 

- Further, if daughter of A has registered a relinquishment deed for release their shares after receiving the amount , then only sons have right to sell and partition the property , otherwise the daughters can claim their respective share even in the absence of a valid document. 

- However , during the life time of B , his legal heirs have no right over the property