What is the validity of an unregistered sale deed?

I have purchased a resale flat in Pune. I have entered into an Agreement to Sale with the owner and we have registered it at the Sub-registrar office. The owners of the flat are a Son and his mother and both were present during registration. The flat was mortgaged with one bank and in my Agreement to Sale, I have mentioned all the details pertaining to my own contribution (cheque numbers etc) so as to smooth the process of the bank releasing the documents. I am also availing a home loan for the rest of the amount from another bank. I have a sanctioned home loan. I was planning to register a Sale Deed and then get the disbursement from the bank. But now the Seller's mother who is also the co-owner, temporarily ill and hospitalized. What can be done ? 1) If my Agreement to Sale is registered, do I need to register the Sale Deed ? If Sale deed is not done, does it hamper the legal validity ? 2) Does subregistrar do a home visit ?