Most daughters would be proud of a mother on the verge of a historic election victory, but the prospect brings pain and dread to Zoilamérica Narváez Murillo, who says she fears the politics of her nation are becoming as destructively incestuous as that of her family.

Narváez is the outcast child of Nicaragua’s first lady, Rosario Murillo, who is standing for the first time in this Sunday’s election on the same ticket as her husband, the veteran Sandinista leader and national president Daniel Ortega.

Polls suggest the first couple will win by a landslide, prompting comparisons with the Underwoods in the TV drama House of Cards, outrage among an opposition who claim a rigged poll is cementing the power of an authoritarian dynasty, and despondency in their daughter.

Narváez claims Ortega, her stepfather, sexually abused her from the age of nine years old. When she went public with these accusations in 1998, her mother turned against her and led a campaign of denials that propelled the president’s wife into a more powerful position both in her marital relationship and national government.

In the years since, Murillo has moved from the fringes to the centre of Nicaraguan politics. She is already de facto chief of staff, the main spokesman of the administration and a micromanager whose approval is needed for every ministerial interview or policy draft. The vice-presidency will formalise her power. According to Narváez, it will also complete a cynical trade-off.

Rosario Murillo and Daniel Ortega at an event commemorating the 36th anniversary of the Sandinista National Liberation Front withdrawal to Masaya, in Managua, Nicaragua. Photograph: Esteban Felix/AP

“I feel like I was sold, that I was part of a transaction. Since I came out and revealed what happened, she has tried to eliminate the truth. It’s part of an agreement she has with Daniel. She told him, ‘Don’t worry, I’ll deal with it’,” Narváez told the Guardian. “She denied the sexual abuse of her daughter to prove her loyalty and maintain power. It was politics that motivated her.”

Ortega and Murillo did not respond to requests for an interview, but they have previously denied Narváez’s accusations and argued that political opponents are trying to use them to undermine a popular government.

They are by no means the only allegations against the first couple, who have also alienated many former associates with their increasingly authoritarian leadership.

Ortega is accused of using his influence over the courts and the electoral council to overcome constitutional term limits for himself and to block the opposition alliance that came second in the last election – headed by the Independent Liberal party – from fielding their chosen candidates. Despite concerns about previous elections in 2008 and 2011, the president has ignored reform recommendations by monitors from the European Union and Organisation of American States (OAS).

“This is the moment of total collapse for the Nicaraguan democratic system. In the past 40 years, we have not seen an election that is less credible or more fraudulent,” said Dora María Téllez, founder of the breakaway Sandinista Renovation Movement, which was earlier stripped of its legal status. “Ortega controls the entire process – who can contest, who monitors the vote and how the count is organised.”

Like Ortega, Téllez risked her life for the Sandinista revolution. In 1978, she was among the leaders of a daring mission that seized the national assembly and took 1,500 people hostage, including most of the congressmen. This was a turning point in the struggle against the dictatorship of Anastasio Somoza Debayle and she hoped it would usher in a period of greater freedoms and social justice. But under Ortega, she says the country appears to be going backwards to an old system of a family dictatorship.

A man cycles past a Sandinista National Liberation Front (FSLN) local headquarters in Masaya, Nicaragua. Photograph: Rodrigo Arangua/AFP/Getty Images

Téllez said that as well as nominating his wife as vice-president, Ortega has ensured the first couple’s children have dominant positions in the tourist industry, the media, and business with China (including over the currently stalled plans for a canal), while their in-laws run the police and oil distribution networks.

“The family of Ortega and Murillo have taken over the Sandinista National Liberation Front (FSLN) like a parasite, using their control to benefit their family. It is basically the same as what the Somozas did,” she told the Guardian, as a tropical rainstorm lashed the streets of Managua, the country’s capital. “He’s a representative of big capital. He and his family are now among the richest in Nicaragua. This is a profoundly corrupt system that completely betrays the principles of the Sandinista movement.”

But many voters remain loyal to the FSLN, which introduced free healthcare and education and more recent benefits – such as free Wi-Fi in city parks – that other governments failed to deliver to one of the poorest countries in Latin America.

With decades of experience, the first couple have also proved among the wiliest political operators in the region, building alliances with key sectors of society, pursuing business-friendly policies, and ruthlessly applying a strategy of divide and rule for their enemies, all the while ensuring an ever tighter grip on all branches of government and the election process.

The powerful Catholic and evangelical lobbies are happy the government has committed itself to strict anti-abortion laws, devoted public funds to church renovation, and that the first couple are running under a slogan of “Christian, socialist, solidarity” – which can be seen on pink and purple roadside banners across the capital. Corporations like the government’s orthodox macroeconomic policy, relatively low crime rates compared with most other central American neighbours, and a GDP growth rate that has been ticking along at about 4.5% for the past two years, despite the sharp decline in financial support from Venezuela and a fall in commodity prices.

The result is some way from socialism or democracy, but it has proved resilient. While other leftist governments in Latin America have been pushed from power, lost key elections or come under pressure in the past year, Ortega and Murillo remain far ahead in the polls.

With the opposition denied or divided, there is no real contest so the strongest vote against the government is likely to come from the campaign for abstentions and deliberately spoiled ballots. It will be hard to know how effective this proves, however, because the result will be easy to manipulate without independent monitors.

Voters have remained loyal to Daniel Ortega and the Sandinista National Liberation Front. Photograph: Esteban Felix/AP

Concerns about the vote may lead to repercussions. The OAS has said it will issue a report on the electoral system six months from now. The United States Senate is also debating a bill – the Nicaragua Investment Conditionality Act (more widely known simply as NICA) – that would impose financial controls on the country if it restricts multi-party democracy.

She’s the only one who talks. You don’t hear other ministers. You almost never hear Daniel. Maria Fernanda Flores

But any punishment would come too late to affect the outcome, which means Ortega is almost certain to secure his third consecutive term, with Murillo becoming his vice-president. How much further she can go is now the subject of speculation, fired by unsubstantiated rumours her husband is unwell.

Some gossip that the old guard of the Sandinistas are manoeuvring to block her from taking over if Ortega died. Others believe Murillo – a poet – is already in the driving seat, due to her increasingly prominent role. It is her voice that updates the nation with a radio broadcast each lunchtime. It is her artistic tastes that have resulted in Managua’s roadsides being adorned with numerous luridly coloured steel “trees of life”.

“I think she is more powerful than him,” said Maria Fernanda Flores, a Liberal opposition congressional candidate and wife of the former president Arnoldo Alemán. “She’s the only one who talks. You don’t hear other ministers. You almost never hear Daniel. You forget what his voice is like.”

Narváez, however, remembers. She said she was driven to exile in Costa Rica by the harassment she faced after her accusations, and that she cannot forget the violence in her family and the lies that followed so they could maintain power. She warned the country could be in for something similar.

“The abuser of a child captures, isolates and brainwashes their victim. That is what is happening to Nicaragua, which has a political system controlled by one family that denies opposition,” she said.