Sorry for your loss.
For 2018, you could claim your mother as a dependent if you provided more than half of her support and she had income of less than $4,150 in 2018. Social security benefits do not count as income.
To claim her as a dependent for past years, you'll need to amend your past tax returns from 2015 to 2017. Returns before 2015 are no longer amendable.
Please see the following TurboTax FAQ for instructions on how to amend your tax return in TurboTax.
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