My father died owning a large number of shares which stepped up in basis on his death. In 2020 my mother sold a about half the shares but erroneously reported the original purchase price as the basis of the sale. By not reporting the Stepped Up basis she reported a much larger capital gain than she should have.
Now three years later the tax return cannot be corrected.
The question I have is can I adjust the basis of the remaining shares she owns; deducting the basis she reported in 2020 from the date of death basis, for the remaining shares.
A capital loss would be result.