John Thuesen has been found guilty of capital murder in the March 2009 deaths of Rachel and Travis Joiner.
The Brazos County District Court jury of eight women and four men took about three hours Thursday afternoon to reach their decision.
Reaction in the courtroom when the verdict was announced was silence. Thuesen, who was standing, did not change his posture.
After the foreman announced Thuesen was guilty of capital murder, District Judge Travis Bryan polled each juror to confirm the unanimous verdict.