• Cancellation of sale agreement

I have entered into an agreement on 26th june 2017 for purchase of a property for Rs53.00 Lakhs against which I have paid an advance of Rs.2.00 lakhs and balance amount within 90 days.It was not a registered agreement.
The seller is an old widow lady having three married daughters.The old lady is the sole owner and all the papers are in her name only.
The agreement was signed by the old lady as well as her three daughters.
The conditions in agreement were kept very aimple for the ease of these ladies.
Only condition was that if the purchaser finds any default after the agreement then the seller has to return the advance money to the purchaser.
Now after getting my loan sanctioned based on the agreement and incurring all the legal expenses so far for the loan as well as advocate charges I approached them for fixing the registry date but now they want to exit from the agreement and dont want to sale the property to us.
We also found that they havent yet encashed the DD of Rs.2.00 lakhs till now which clearly shows that they were aware of any such situation but didnt even let us know earlier.Even nearby people also said that they are still visited by other purchasers so it might be that they are aelling to some other parties which breaches the agreement clause.

As there was no default clause in the agreemnt can I take seek of law for not getting this agreement cancelled.??
I have already proceeded too much for buying this property and not in a position to back off right now..
Please suggest what actions or measures can i take to purchase the property by the help of law.