As I see, the picture about the said property is:
1) Seller's Father - got one share
2) Seller's Uncle 1 - got one share
3) Seller's Uncle 2 - got one share
4) Seller's Aunt - no share
And the said property is inherited not self-acquired by any of them, as you said, The Seller's Father, His Uncle 1 and 2 distributed property among themselves giving no share to The Aunt! It seems the property was 'intestate'. Therefore, all three brothers and one sisters should have got equal shares. The three brothers had no right to distribute the intestate property among themselves without dividing the share of their sister. And the partition should have been registered!
If they intended not to give share to their sister, then they should have had taken consent in writing in partition-deed by the Sister. Therefore, the partition seems invalid and sister can still claim partition! As intestate property was not properly divided, Seller cannot sell the share of property his father occupies! Even the father is not owner of his share, because no one has Registry of their respective shares, i.e. Seller's Father, His Uncle 1 and 2. They are just living on property by mutual consent!
Just mutual consent cannot make owner of property and cannot give right to sell. I think, first Partition Deed should be made of the property and property be distributed among Seller's Father, Uncle 1, Uncle 2 and Aunt! Once registered, father can transfer his 1/4th share to his son, i.e. the said seller! And then the seller can sell the said portion to you. If Sister, i.e. Seller's Aunt, gives consent in writing of her waiver of all rights in the property, then property can be divided among Seller's Father, his Uncle 1 and 2. Then after seller's father transferring his 1/3rd share to his son, i.e. said seller, then seller can sell you the share.
There can be no Oral Partition of immovable properties! Therefore, no partition took place in 1994!
ANSWERS to your QUESTIONS:
1) Yes, she still has right in the property and she can claim partition and her share from all persons ocupying the property.
2) Any kind of document getting signed from him doesn't make sense and will serve no purpose in case his aunt claims partition or her share! This is not at all a practical solution!
This was all I could tell you according to my knowledge, hope this might prove helpful to you!
Seek advice of Learned Counsel in your city and then proceed with the transacton!