Dear Sir,
Generally Income tax return is filed for an assessee who has income exceeding the taxable income as specified by the income tax or if there is a refund to be claimed by the assessee even if the income is below the tax slabs.
As the income of your mother is below tax slab and since you haven't mentioned anything about tax deducted at source there is no need to file income tax return.
If, there is a refund claim then you need to file the income tax return. And even if you file the return post 31 st July,2018,as per section 234F the penalty for late filing of income shall be Rs.1,000 if the income is below 5,00,000.
Also there is no need to include her income in your income. You can revise your return and claim the tax paid if any as refund.
Hope this resolves you queries.