Purchasing gift deed property

I visited a property (House) for purchase which is owned by a lady and I came to know that it was gifted to her by her mother-in-law as their share of the property. Gift deed is executed in Sub registrar's office with two witnesses. Now Son and daughter in law are planning to sell their house as they have mortgaged property and in need of finances. IN the course of purchasing, I happened to meet her father-in-law and mother-in-law who stats next door who has written the gift deed, and they are against selling the house. They threatened that this gift deed was done under coercion as the lady is not in good mental health and an alcoholic, hence they are going to put a case to revoke this deed on the basis of this, hence warned against purchase of this. Owner of the property says there is no risk in buying this as the gift deed has stated that she has done the gift deed willingly and they dont have any rights on the property. They agreed that they have done the gift deed but she was only to write a will , but somehow they used her naivety to make this as gift deed. I am interested in purchasing this property, but I am worried about getting into cases. Though in gift deed, I see that it is clearly mentioned that Daughter in law is solely responsible for the property and she can do anything with it. My question is : What are the chances of the mother-in-law going to court and getting the gift deed revoked successfully? Is it risky to buy this property?