Whose income tax return are you preparing and for what year?
Your parent's last income tax return (2018) should have been submitted by now, and if some stock was sold by your mother between the time of your dad's death and her passing the situation should be straight forward.
If your mom inherited dad's stock and you inherited your mom's stock then the "non-community property state" issue should be moot. All the stock you inherited would have a basis dictated by the fair market value of the stock on the date of her death.
