Also note that for 2020, if your mother in law did not receive the round 1 and round 2 economic stimulus payments ($1200 and $600), a tax return could be filed in her name to claim a rebate in the amount of the missing stimulus, even though she has no taxable income. To file a return in her name, someone would have to have a power of attorney, or be appointed by a court as her legal representative. The same thing is true for the 2021 stimulus of $1400, if she did not get a check or a deposit into her bank account, it could be claimed on a 2021 tax return.
It would not be necessary to "catch up" by filing 2018 and 2019 returns in order to file a 2020 return.
