Purchase of Open plot

Dear Sir, I would like to purchase a open plot whose owner is died on 18-11-1998. The deceased plot owner is survived by his mother, wife, two married daughters and one married son and the deceased plot owner’s father is expired. On 16-06-2012, the said plot was registered by way of Gift Settlement Deed in favour of said deceased plot owner’s two married daughters by his wife and one married son by producing the death certificate of the deceased plot owner (his mother is not involved in the registration and as I already said father is expired). The two married daughters and one married son are having minor children now (all are about having below 8 years old). The deceased plot owner’s mother is living separately and is not having good relations with the deceased plot owner’s other family members i.e. his wife, daughters and son. Can I purchase the said plot. Are there any legal problems in future from the deceased plot owner’s mother or from her grand children or from any other angle? If the deceased plot owner’s mother also signs at the time of registration to me as witness is it ok. Please advise. After enquiry it was known that the deceased plot owner’s mother is also having share in the said property. But by the time I am knowing the above, I paid Rs.15000/- in cash to the deceased plot owner’s two daughters (who are presently the owners of the plot) and got an agreement paper signed by them. In the agreement it was written that they will receive Rs.1,56,875/- on 19/10/2014 and remaining balance on 5,15,625/- on 27/11/2014 and got the registration done in favour of me on 27/11/2014. But as I said after payment of Rs.15000/- I came to know through Lawyers Club India that they are not absolute owners of the plot, so, I requested them to get the signature of their father’s mother also at the time of registration. But they have not agreed for that Then I have not paid the amount of Rs.156,875/- on 19/10/2014 and alo requested them to refund already paid amount of Rs.15000/- then I will return the agreement paper signed by them. Number of times I called them over phone and also asked in person to do it. They are saying that they will refund my money and take the agreement paper back, but they are not telling the date on which they will refund my money and take paper back. Now the date of registration which is on 27/11/2014 is near. Now what I have to do ? Either I should issue a notice to them before 27/11/2014. I am ready to loose my Rs.15000/- but if they issue a notice after 27/11/2014 that I have not paid the balance amount and got registration done can I tell them since they are not having full ownership over the plot that’s why I have not paid the balance amount and got registration done. Waiting for their notice is also legally binding me ? Or As per law I should issue a notice to them before date of registration ? But if I keep quite till their notice is there any problem. Please advise.