Grandfather passed in Feb 2019, been living in home since. Mom trying to sell house. Can she? No will.

If there is no Will, the estate must still be settled.  If your mother has been appointed administrator by the court or a court has approved a Small Estate Affidavit or a title company recognized an Affidavit of Heirship, she can, and in the first two cases has the duty to, sell the house, pay the debts and distribute the remainder according to the Texas intestate rules. Time to move.