She smiled while testifying against her own sister who is facing a murder rap, but Dona Cantrell-Robinson wasn't moved to tears until it came time to rehash her own past.

Cantrell, the sister of Margaret Rudin, withstood a heated cross-examination, but broke down for the first time in her three days on the stand when prosecutors asked her about antidepressants she took as a result of a physically abusive relationship.

The soft-spoken blonde told Judge Joseph Bonaventure she needed to take a break when prosecutor Gary Guymon asked her to explain to the jury why she was taking prescription antidepressants in 1989.

Following the break the prosecution witness said that she was severely beaten by a man she was involved with, causing her to pack up with her son and move 2,000 miles away.

Just minutes before, Cantrell had worn a coy smile as defense lawyer Michael Amador attempted to paint her as a woman who wanted to hurt her own sister.

Amador (Court TV)

The lawyer — who last week said he agreed with Rudin that he was inadequately prepared and unsuccessfully sought a mistrial — grilled Cantrell about her motivations for testifying against Rudin.

"You'll do anything to hurt your sister. The only other thing you like is the spotlight and money," he said.

Amador asked if she agreed if it was "unusual" for a sister to cooperate with authorities prosecuting her sister for murder. Cantrell admitted that she spoke to detectives and even mailed a photograph of a trunk prosecutors contend was used to dispose of the body of Ron Rudin.

But Cantrell appeared unflappable, at times trying to squeeze in explanations to yes or no questions, agitating Amador and prompting a mild admonishment by Bonaventure.

"My motivation was purely about the murder," she said. "I knew what I knew, I saw what I saw, I heard what I heard."

Amador then quizzed her about an appearance on the tabloid television show Hard Copy, for which she was paid $2,000.

Amador reminded her that at about the same time she mailed the photo of Rudin taken inside her antique shop before Ron Rudin's Dec. 18, 1994, disappearance — which depicts a trunk in the background — Cantrell asserted her right not to testify in a civil trial about Ron Rudin's estate. The proceeding was the result of Margaret Rudin's claim for her share of her husband's $11 million fortune.

The charred remains of the trunk found near Ron Rudin's skull were admitted into evidence earlier in the trial (Court TV)

Cantrell responded that if she had been interested in the money, she could have aligned herself with her sister or even the trustees battling the widow in court.

"I did neither one," she said.

"I had a view about who committed that murder, so fighting about money didn't fit into my value system," she added later.

The civil trial, which ended long before criminal charges were brought against Margaret Rudin, ended with a settlement of $600,000 for the widow, just enough to cover her legal expenses.

"I was being used by both sides and it had a detrimental effect on my life," Cantrell said.

That comment touched off a fiery reaction by Amador, who accused Cantrell of only doing things that would directly benefit her own life.

The prosecutor Guymon may have inadvertently highlighted that point during his redirect examination. When he encouraged Cantrell to talk about her abusive relationship, Cantrell was emotional to the point of tears — emotion she had not displayed for her sister on the stand.

During re-cross, Cantrell seemed more somber than during previous exchanges with Amador, who showed no mercy toward the distraught witness, even calling her "poor, poor Dona."

Amador accused her of being a "problem," the "bad sister" and the "troublemaker" of her family, charging that Cantrell had beaten Rudin and that even their mother didn't like her. The witness denied those allegations.