Declaring inheritance?

Hi,

 

My mother died in 2019, and in 2020 I received an inheritance. My brother, the trustee of her accounts, indicated that we did not have to pay tax since the amount of the inheritance fell under certain limits.

 

However, what are those limits, and is there some form we need to file to declare the money received?

 

I already filed my 2020 taxes, and only included investment gains on the moneys in Schedule C. If I need to make amends, what should one do?

 

Thank you.