Former Virginia governor Robert F. McDonnell’s sister took the witness stand Tuesday at his corruption trial to make the case that his marriage was far more strained than his finances.

The assertion is key to the defense of McDonnell and his wife, Maureen, who say that their relationship was too troubled to support a conspiracy and that McDonnell would never stoop to unethical behavior for money.

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The couple has been charged with trading the prestige of the governor’s office for $177,000 in luxury gifts, vacations and large loans from Jonnie R. Williams Sr., a dietary supplement executive.

Defense attorneys, who are presenting their case, want to convince jurors that the former governor was a trusting public servant who promised Williams nothing and was essentially duped by a shady salesman and even, at times, his own wife.

List of gifts given to the McDonnell family from Jonnie Williams.

While jurors had heard similar testimony, McDonnell’s youngest sister was able to offer a personal perspective, at times detailing messy internal dynamics of a family that had remained hidden despite the public spotlight in which they lived.

Maureen C. McDonnell, who shares a first name with the former first lady, co-owned investment properties that prosecutors allege were secretly draining the governor’s finances to the point that he sought large loans from Williams.

She testified that she has observed her brother’s relationship with Maureen McDonnell for the past 40 years. Maureen C. McDonnell said she was a child when she met the woman who would become her sister-in-law.

“There are two sides to Maureen,” she testified. “You’re not sure which one you’re going to get — which one will show up.”

One, she said, was “very sweet, very tender.” She recalled how the former first lady sat with her husband’s mother as she lay dying, rubbing lotion on the elderly woman’s legs and head.

But, Maureen C. McDonnell added: “She could be very manipulative. She could be very unpredictable. She could be very deceptive.”

Over the years, the couple had ups and downs, but McDonnell’s sister testified that their communication went from “bad to worse” after he was elected governor. Maureen McDonnell appeared anxious and uneasy in her role as first lady. Once she texted her sister-in-law asking to meet before she had to return to what she referred to as “that prison-mansion.”

In the spring of 2012, after an insulting quip from her brother’s wife left her in tears, McDonnell’s sister testified that the former governor told her he was trying to work with his wife. “He was trying to get her help,” Maureen C. McDonnell testified.

Although at times uncomfortable, the defense strategy could generate sympathy for the former governor and his wife. More important, it would be difficult to convict either McDonnell on public corruption charges if jurors are convinced that the first lady, who was not a public official, acted to promote Williams’s company without her husband’s knowledge.

In March 2012, for example, Maureen C. McDonnell testified, she returned from a vacation in Jamaica to find a mysterious text message from her sister-in-law about a $50,000 check that had just arrived — a check that jurors now know was a loan from Williams to the governor’s real estate company.

Maureen C. McDonnell said that in a text and in a conversation later with her brother, her sister-in-law seemed to be claiming responsibility for having arranged the loan — and seemed to be upset that she was talking to her brother about it. Maureen C. McDonnell said that she called her brother to discuss the matter, and that she could hear the first lady screaming in the background.

“She was livid,” McDonnell testified. “I just heard some of the narrative that we heard earlier, that she worked on the loan, not Bob.”

The secrecy of the first lady was reinforced by another witness presented Tuesday by defense attorneys: Kathleen Scott, who served as a special assistant to Maureen McDonnell.

While trying to do some organizing for her boss in the fall of 2011, Scott testified, she found a Rolex watch in a vanity filled with jewelry, boxes, handbags and lipstick. It was not in a box and had no accompanying paperwork. Trying to be helpful, she put the watch in a box and placed it on a high shelf.

At first, Scott testified, the first lady was pleased with her efforts. But later, she became angry. She said she could not get ready for a major event, the annual Thanksgiving tax tribute ceremony, because she could not find her things. And she accused the aide of taking the Rolex, Scott said.

Jurors know that Williams bought a Rolex in August 2011 and that Maureen McDonnell gave it to her husband for Christmas. Whether he in fact knew the gift came from the wealthy benefactor is in dispute.

Like other aides, Scott, whose children had played soccer with the McDonnell children before she went to work in the mansion, testified that the first lady could be an anxious and controlling boss. A management consultant brought in to improve working conditions for employees advised them to act as though they were “dealing with a 5-year-old,” she testified.

When discussing Williams, the first lady would “light up,” Scott testified.

“She seemed enamored with him. Infatuated. I think he made her feel special,” Scott said, supporting a defense theory that Maureen McDonnell had a crush on the executive and sought his attention, not his money.

When not exploring the former first lady’s behavior, defense attorneys spent much of the trial’s 17th day on a drier subject: finances.

McDonnell’s sister offered extensive testimony about her own finances, saying that she made nearly $530,000 a year as a vice president for health insurance giant Amerigroup while her brother was governor and that she had significant assets. She co-owned MoBo Real Estate Partners with her brother, which managed their rental properties and was extended $70,000 in loans from Williams in 2012.

Despite e-mails presented by prosecutors in which she wrote of financial pressures and appeared to contemplate bankruptcy, Maureen C. McDonnell said she and her brother were never desperate.

She testified that she had not “ever experienced financial pain” in her adult life and said that if her brother needed money, he knew he could turn to her. Indeed, McDonnell’s sister testified that she tapped her own funds in July 2013, when the governor publicly apologized for his relationship with Williams and announced that he had repaid the executive in full.

“It was a couple of phone calls,” she said. “It was rather painless.”

At day’s end, defense attorneys revealed detailed financial information about the governor, his sister and their real estate company, suggesting the outlook was far rosier than prosecutors had said.

Together, the two had almost $1.4 million in liquid assets, including stock and retirement accounts, said J. Allen Kosowsky, an accountant who reviewed their information for the defense. The credit card debt the governor shared with his wife — which at one point was about $90,000 — was below $6,000 in August 2011, Kosowsky said. And even the governor’s real estate company seemed to be covering its expenses with rental income by some point in 2012, he said.