Seller dont have patta in his name for the property by settlement

Whether the settlement documents were registered documents or just unregistered deeds?, if unregistered deed of settlement, it is not valid. What about the encumbrance certificate, whether the settlement entries reflect in it?
How did the mother acquire the property?
Is the seller having the original parent deed in his possession?, have you seen it?
What was the necessity for a second settlement deed in the same family within a span of two years?
Whether the person who settled the property in favor of his brother, the seller herein, is a married person with children ?
Why did the patta and chitta remained not transferred ?
Where is the question of Chitta adangal for residential land?
By not considering the settlement at all as a matter (if they were registered), you can get the patta transferred on your name from the seller's mother's name.
You said that other documents are clear, what are the other documents?
You need to have a proper legal opinion by an expertise lawyer who shall render his/her opinion after proper perusal/scrutinization of all relevant documents that you may produce or he may desire to see.
It is a matter of one time investment so spending few thousand on legal opinion will secure your investment hence do not hesitate to have a proper legal opinion from a good lawyer.