GPA from Seller to Builder

Dear Sir, We are planning to purchase an apartment from a builder who has built the apartment on a sold land. The land belonged to a Mr.X who has divided the land between 4 of this children in parts (schedules). The schedule 3 and schedule 4 belong to three of the children of X - A, B and C. This is the part of the land on which the apartment is built. The seller side of the property on the sale deed has 6 parties. Parties 1-3 are A,B and C respectively and have their signatures on the sale deed. But the sale deed mentions names of children of C - namely P,Q and R as parties 4,5 & 6 respectively. The father of the children (C) has signed as the GPA on the sale deed on behalf of P,Q & R. We have asked the builder for the GPA document between the parties and he has provided an e-stamp copy with notary sign of all the 3 GPA's of P,Q & R with their father. One of the child (R) resides in USA and hence may not be able to travel to India to sign in person, hence has a India Consulate of USA sign on the GPA papers.This is a specific GPA and mentions the purpose as the sale of the land. But the GPA is not registered in the Sub- Register's office. Will this non-availability of the registered GPA cause any issues in the future? Also the bank has provided loan to this property. Approx 95% of the apartment is already occupied. Please do help understand the risks this non- availability may cause.