This is not a TT/income tax issue. The estate/gift tax regime is different. Receipt of a gift is not income. https://www.law.cornell.edu/uscode/text/26/102
This is very complicated and you should seek professional guidance from an estate planning attorney.
Basically everyone is required to annually report (on a Form 709 gift tax return) all gifts they make over $14k (with some exceptions for medical, educational, etc. paid directly to the providers). When the total of these gifts exceeds the current exclusion amount (~$5.5M) a gift tax will be due. When a person dies and the value of their estate plus lifetime gifts (over the $14k/yr) is > $5.5M, they have to file an estate tax return (Form 706). They may have deductions and not actually owe tax (e.g. amounts passing to a spouse).