First, an application for condonation of delay to be filed. For more info:
1) https://cleartax.in/s/how-to-file-income-tax-return-for-last-years
2) https://www.caclubindia.com/articles/filing-income-tax-return-after-due-date-44429.asp
First of all, if your (means mother's) income is below taxable, than she is not required to file any income tax return.
If you want to show Cash income than you can calculate cash income for that three/four year and can deposit or shown as cash in hand in you books. and now and onward file your return regularly.
As I have already said Best option is to You have to file return under condonation of delay u/s 119(2)(b) from 2017 by showing sufficient reasons for not filing returns
If there is genuine need then you should apply , if in all the years income is less then basic limit, then you just leave it. And also if there is any income for which return filing required then you apply.