The mother of Alberto Sierra testified Tuesday that she loves her son but does not excuse his actions, as jurors considered whether he should be sentenced to death for the 2012 killings of his estranged wife and her two young daughters. “Alberto is my son; I’m not justifying what he did. I’m not making up any excuses, but he is my son,” Mercedes Pedraza told the court during the penalty phase. Sierra, 42, became emotional as she took the stand. Sierra pleaded guilty in August to killing Gladys Machado, 29, and her daughters, Julia Padrino, 6, and Daniela Padrino, 3. He suffocated all three with a plastic bag. A jury will decide whether he receives the death penalty. Pedraza described her son’s upbringing, relationships and behavior after he was struck by a car at age 12 and suffered a traumatic brain injury. She also recalled going with him to the police station after the killings. She testified that he reassured her, saying everything would be OK, before she hugged and kissed him. The defense argues the childhood injury affected Sierra’s behavior and decision-making. Dr. Erin Bigler, a psychologist who specializes in brain and behavior, testified remotely as jurors viewed Sierra’s brain scans. Bigler said the frontal lobe can be especially vulnerable in a vehicle impact, and that a person’s brain may sustain additional injury when the body hits the ground. The penalty phase was scheduled to resume Wednesday morning.