As other members have suggested, please talk to the concerned HR and also your boss. The next step is to mail the scanned copies of the medical reports, bills, etc., to your boss and HR, clearly stating your request for an extension of NP.
As long as HR and your boss are convinced that this is a genuine case and not some made-up story for any reason, which can be achieved by speaking the truth and presenting all the facts, you shouldn't worry.
Clarification Needed
What does this line mean, "so it may be quite a while before I can get back to putting in the documents"?
What is being suggested in this forum is to submit the relevant medical bills for your mother's treatment. I am sure you are with your mother, right?
If you are referring to the submission process to Wipro, you already have an alternative. You can email them after discussing and explaining over the phone.
Also, inform your new company about the situation causing a delay in your relieving. They need to be prepared for your delayed joining.
By the way, not being registered with NSR has nothing to do with this case.
Regards,
TS