It's been five years since Nujood Ali became known as the world's youngest divorcee after escaping the man who bought her as a child bride aged nine.

The story of Nujood's marriage and subsequent court victory was turned into a bestselling book, bringing hope to thousands of Yemeni brides forced into marriages they are too young to understand or consent to.

The royalties from I Am Nujood, Age 10 and Divorced were supposed to pay for the girl's schooling and allow her to follow her ambition to become a lawyer. Instead, Nujood says, the money has been squandered by her father – who has now sold her younger sister to a man twice her age.

"My father has spent all the money on getting married twice again," she says, fidgeting nervously, her fingers stained with henna.

Now 15, she still finds it difficult to talk about her marriage and ex-husband. "He now has four wives, 14 children and learnt nothing from my experience. He gives me between $20 [£13] and $30 a month for pocket money."

Nujood's ordeal began when she was married off by her father at nine, for a dowry of a little more than $750, after her future husband, Faez Ali Thamer, promised not to have sex with her "before the year after she has her first period" – as required by law in Yemen.

But what followed was a cycle of sexual and physical abuse, starting on her wedding night.

Two months later, during a visit back to her family home, she took the unprecedented step of running away and asking a court for a divorce on grounds of abuse.

The case was the first of its kind in Yemen and attracted so much attention that the court's security described the hearing as a "mob scene". Both Nujood's father and husband were briefly imprisoned during proceedings, after colluding to lie to the court about the girl's age.

The book of Nujood's story was ghostwritten by Delphine Minoui and published in France. It was translated into 16 languages and sold in 35 countries.

Publishers Michel Lafon agreed to pay her father, Ali Mohammed al-Ahdel, $1,000 a month until she was 18 to support her upbringing. It also bought a large house for the family in Sana'a, and set up a fund paid directly to a school for her education.

But Nujood says she has been forced out of the home and has not received any of the money being paid to her father. She said her father had rented the first floor of the house to another family, and moved his new wife into the second. "I've been asked to leave and have to stay in my older brother's cramped house."

Listening carefully in the corner of the room is Haifa Ali, Nujood's younger sister, who recently became engaged to a man she does not know.

"I don't want to get married," Haifa says. "I'm very scared, because the [dowry] money has already been paid and I want to continue my education."

Haifa is cut short by Nujood, her anger overcoming her shyness. "I won't let it happen to her," she says. "I will speak to as many journalists and lawyers as possible about this. It is illegal."

The girls' father refused to speak to the Guardian, but the book's publishers say they are trying to rectify the situation. "We are unable to pay Nujood directly legally in Yemen due to the law and it is at times exceptionally difficult to know what is going on from France," said Margaux Mersie of Michel Lafon.

"The problem is that al-Ahdel's position is upheld in Yemeni law. There are plenty of judges who support him and are unsympathetic [to Nujood]."

Nujood's attitude is best summed up by her reaction to being granted a divorce. She still wishes to study in England and become a lawyer.

"Compared to dreams, reality can be cruel," she said. "But it can also come up with beautiful surprises."