The sister of a teen convicted of killing his grandparents spoke out to FOX News Wednesday and criticized the jury's guilty verdict.

Christopher Pittman's sister, Danielle Pittman Finchum (search), said that her brother is not guilty and the family will not give up the fight to get him released.

“I know in my heart my brother is not guilty,” Finchum told FOX News. “In his current state of mind he would have never have dreamed of hurting nana and pop-pop.

"We're going to fight for him and if I have to keep fighting for the next 29 years to get him out, he's not going to stay in there the full time that he's supposed to," she said.

Christopher Pittman (search), 15, was convicted Tuesday as an adult of two counts of murder and sentenced to 30 years for killing his grandparents with a shotgun when he was 12 years old. Pittman's defense maintained the anti-depressant Zoloft (search) made him manic and he couldn't tell right from wrong.

Family and supporters contend that Pittman was too young to tell the difference between right and wrong. They also blame Zoloft for being a “mind altering drug.”

“It turned him into something he was not,” Pittman’s sister told FOX News.

His age also troubled jurors who debated whether he should be convicted as an adult.

"We don't intentionally hurt the ones we love. But I have an adult brain and you try to think about what a child was thinking," Christine Peterson, 54, of North Charleston, a juror who is the grandmother of a 12-year-old.

"I'm sure many of us had sleepless nights," she added.

The prosecution countered he was simply angry at his grandparents for disciplining him for choking a younger student on a school bus. Following the slayings, Pittman burned the couple's Chester County home and drove off in their car.

"If Chris Pittman had been 25 we could have come to a decision much, much earlier. Because of his age it was very, very difficult," said Arnold Hite, the jury foreman and a Charleston Southern University professor who said he was speaking for himself.

Pittman was sentenced to 30 years on each count. Circuit Court Judge Danny Pieper ordered the sentences to run concurrently.

Pittman hung his head as the verdict was read.

"I know it's in the hands of God. Whatever he decides on, that's what it's going to be," Pittman told the judge. Thirty years was the minimum sentence Pieper could give; the maximum was life.

Pittman's father, Joe, told Pieper he supports his son even though the victims of the November 2001 shootings were Joe Pittman's parents.

"I love my son with all of my heart, as I did my mom and dad," he said. "And mom and dad, if they were here today, would be begging for mercy as well."

Later, Joe Pittman called on South Carolina Gov. Mark Sanford (search) and President Bush to pardon his son. He said he is a Gulf War veteran and urged the officials to "give my son the benefit of the doubt."

Prosecutors said they didn't think the Zoloft defense was viable. "I really think that was a smoke screen," prosecutor John Meadors said later. "He just happened to be on an antidepressant when this happened."

Christopher Pittman initially told police that a black man shot his grandparents, burned down the house and kidnapped him.

Prosecutor Barney Giese reminded jurors how the boy carried out the killings, saying, "I don't care how old he is. That is as malicious a killing — a murder — as you are ever going to find," Giese said.

He pointed to Pittman's statement to police in which he said his grandparents "deserved it."

Pfizer Inc. (search), the manufacturer of Zoloft, said in a statement after the verdict: "Zoloft didn't cause his problems, nor did the medication drive him to commit murder. On these two points, both Pfizer and the jury agree."

In April, a Santa Cruz, Calif., a man who beat his friend was acquitted by a jury of attempted murder after he blamed the episode on Zoloft. But in at least two cases last year, juries in Michigan and North Dakota rejected similar claims.

Zoloft is the most widely prescribed antidepressant in the United States, with 32.7 million prescriptions written in 2003. Last October, the Food and Drug Administration (search) ordered Zoloft and other antidepressants to carry "black box" warnings — the government's strongest warning short of a ban — about an increased risk of suicidal behavior in children.

The Associated Press contributed to this report.