Please apply for a PAN first as that is the most important document to start with.
After applying for a PAN you can file your Mothers return and pay the due taxes. Do not worry as it can be done before 31st March 2017 assuming that your last year was FY 2014-15. You will only be charged an interest on the amount of Tax payable.
In case you need any assistance for filing of returns, we will be happy to help.
Trust this clarifies your query.
Feel free to get back/ call back for any further clarifications.
Rohit R Sharma
BCOM, ACA, LLB - GEN, CERT. FAFP