When her widowed mother remarried, Parvin Rema, then 13, was part of the deal – one of several such arrangements in Bangladesh. Abigail Haworth talks to mothers and daughters about a particularly knotty relationship

As a child in rural Bangladesh, Orola Dalbot, 30, enjoyed growing up around her stepfather, Noten. Her father died when she was small, and her mother remarried soon after. Noten was handsome and energetic, with curly dark hair and a broad smile. "I thought my mother was lucky," Orola says when we meet in the dusty, sun-baked courtyard of her family home in the central forest region of Modhupur. "I hoped I'd find a husband like him one day." When she reached puberty, however, Orola learned the truth she least expected: she was already Noten's wife.

Her wedding had taken place when she was three years old in a joint ceremony with her mother. Following tradition in the matrilineal Mandi tribe, an ethnic group of about two million people spread across hill regions of Bangladesh and India, mother and daughter had married the same man. "I wanted to escape when I found out," says Orola. "I was shaking with disbelief."

Disbelief was more or less my reaction a few days earlier when, by chance, I'd first heard about this marriage custom. I was visiting the remote Modhupur region to report a story about Mandi women fighting deforestation. My travelling companion was an eminent Bangladeshi environmentalist called Philip Gain, who had been studying the area for more than 20 years. As we drove through the khaki- coloured hills, we talked generally about how Mandi women were the property-owning heads of their households. Gain, 50, a professorial man in a suit jacket and tie who runs the Dhaka-based activist organisation Society for Environment and Human Development (SEHD), told me how they shared power with men and had far more independence than women in the majority Bengali population.

Then Gain mentioned the mother-daughter joint marriages. He explained that among the Mandi, widows who wish to remarry must choose a man from the same clan as their dead husband to preserve the clan alliance. The only available single men, however, are often much younger men in their late teens. So the custom evolved: a widow would offer one of her daughters as a second bride to take over her marital duties – including sex and child-bearing – when the girl came of age. "It's not common these days," said Gain. "But it still exists among a few Mandi families."

Bangladesh is a deltaic country where most of the 160 million people are Bengali Muslims. It is better known for its flood plains and typhoon-lashed coasts, but its southeastern and central hills are home to ethnic minorities who mainly practised animism until Catholic missionaries arrived in the late 19th century. The Mandi, who number 25,000 in the Modhupur region, live a six-hour drive and a world away from the frenetic capital Dhaka.

Orola is cooking rice and lentils for breakfast on an open fire when we arrive at her hamlet, a cluster of mud houses flanked by scrubby fields. Her family members are all there: her 51-year-old mother Mittamoni, her stepfather and husband Noten, 42, her maternal grandmother and an assortment of children ranging from babies to teenagers, fathered by Noten with both his wives. Everyone is doing household chores in the weak morning sunlight.

The family's marital arrangement is an open secret in this small Modhupur community, but nobody, Orola says, ever mentions it. "For years I wanted to talk to someone about it because I was lonely. But people think it's un-Christian, so they ignore it." Missionaries have converted the majority of the tribe's local population. Traditional rituals, such as sacrificing goats to restore a sick person's health, are frowned on by the clergy and have waned. "Bridegroom kidnapping", another rare custom in which Mandi women abducted potential suitors and held them hostage until their wedding day, has also died out. A handful of mother-daughter joint marriages have most likely survived because, like most unions worldwide involving multiple spouses, they serve an economic purpose.

"My mother couldn't manage her land and household by herself when my father died of fever," explains Orola. "She was still in her mid-20s, so she was entitled to claim a new husband as a replacement from my father's clan." The clan offered their only available bachelor at the time, Noten, who was then aged 17, on the condition he marry Orola, too. Since Mandi marriages represent the consolidation of wealth between two clans, the second, younger wife is a trade to ensure the birth of more children to add to the family's overall wealth and power.

"I was too young to remember the wedding. I didn't know it had taken place," Orola tells me while she stirs her pots. Although such an arrangement is not considered incest or even child abuse in Mandi culture, where early marriage is the norm, she was distraught to discover she was forced to share her mother's husband. "The last thing I wanted was to be married to Noten. I wanted a husband of my own."

The situation was doubly unjust for her, she says, because ethnic Mandi women traditionally choose their own partners. It is women who make the first romantic move, and also propose marriage. Property is passed down the female line, and men live in their wife's household when they marry. She watched her female friends embark merrily on their love lives and felt so isolated that she considered suicide. "I felt trapped, like an animal."

Mandi women returning home from their work in the fields. Photograph: Eric Rechsteiner for the Observer

The three-way marital arrangement was fraught as soon as she was officially a co-wife. "It grew tense when Noten began sleeping with me. My mother knew it was inevitable – she pushed me into Noten's bed when I was 15 to consummate the marriage. But he quickly began to prefer me to her." In a whisper – Mittamoni is hovering nearby – Orola relates how her mother once slipped some wild herbs into her food to upset her stomach. "While I was ill, she took the chance to spend the night with Noten."

The rivalry destroyed their mother-daughter relationship. "She stopped being my parent. I couldn't turn to her for advice any more." Orola rebelled against her new role, taking off on solo day trips to the district capital of Madhupur to go shopping and watch Bengali movies. "I used some of the family money to buy gold jewellery. I knew I'd never have a man of my own to buy gifts for me, so I bought some for myself."

Her resistance faded when she became pregnant. Now she is the mother of three children with Noten: a boy aged 14, and two girls, aged seven and two. Mittamoni has a 17-year-old son with Noten, as well as an older daughter who has already left home. Life is hard and basic, especially since wholesale government plundering of the tribe's ancestral forest has fractured the local economy. Their hamlet has no electricity or running water. The nearest town consists of a row of open-fronted shacks selling rice, cooking oil and candles. Orola and Mittamoni jointly own a few acres of land, from which they make a modest living cultivating pineapples and bananas.

Mittamoni, a gaunt woman with her black hair scraped into a tight bun, listens without apparent emotion as Orola talks. Does she feel guilty hearing her daughter's words? "No, I don't," says Mittamoni. "The marriage was necessary for our family's survival. It was the decision of our clan elders, not mine." She insists that she protected Orola until she was "old enough to be a wife", and that sharing a husband wasn't easy for her either. "I had to step aside when Noten became intimate with Orola, and that was difficult." Noten, who is also present but doesn't want to speak, immediately throws his hands in the air, as if to say, "Don't put me in the middle of this." The gesture is so natural he clearly uses it often.

Orola ignores them both and picks up her young daughter. "Her name is Walni," she says, smiling. "It means 'new dawn' in Mandi."

Little is known of the custom outside Mandi culture. At the local Catholic mission in Pirgacha – the community's social hub – I find a copy of the first-ever study of the tribe, written a century ago by Major A Playfair, a British colonial officer and keen amateur anthropologist. Entitled simply The Garos (another name for the Mandi), the book contains a meticulous inventory of sacrificial chicken rituals but only the briefest mention of the (undoubtedly more unpalatable) mother-daughter marriages. American anthropologist Robbins Burling, who lived in Modhupur for a year during the 50s, also skated over the custom because there were "no cases" in the particular Mandi village in which he was studying.

The greatest local authority on such matters is female elder Shulekha Mrong, head of the local women's organisation Achik Michik (Mandi Women's Unity). Mrong says that clan matters concerning marriage are extremely involved. "We have many kinds of arrangement to safeguard property-owning female lineages. A widow and her daughter marrying the same man is just one of them, and it's hardly practised any more." Still, she believes it shouldn't exist at all. "The custom is a great injustice to young girls. They're denied freedom of choice, and it's emotionally damaging to be in the same relationship as their mothers."

She cites recent cases where young women have bolted from such arrangements, fleeing to Dhaka to work as maids or beauticians. Modern Mandi women, she says, pride themselves on not tolerating any form of abuse. "We don't allow domestic violence or adultery. If a man hits his wife or cheats on her, we make him pay a fine to make amends – a few pigs, or a lump sum of cash. It's a very good deterrent." To avoid violating their daughters' rights, widows should find new husbands their own age, she says, and there should be a system to compensate the dead husband's clan if they lose out financially.

There are three of us in this marriage: Parvin Rema, 36 (far right), with her mother, Joyanti, 59, and their mutual husband, Palnat, 42. Photograph: Eric Rechsteiner for the Observer

Parvin Rema, 36, another local woman who shares a husband with her widowed mother, agrees. Parvin and her mother, Joyanti, jointly married an 18-year-old man, Palnat, when Parvin was 13. "I thought my life was ruined when the wedding took place," says Parvin. "My mother was 36. I didn't understand why she wanted such a young husband." But Parvin quickly used her youth to her advantage. "My mother slept with our husband for the first three years. But when I was old enough I made sure he lost interest in her. I cooked him curries and never refused him sex." Parvin's mother, Joyanti, was soon ousted. Their husband started treating her like his mother-in-law rather than his wife – he was polite but distant – and Parvin took over as the household's alpha female.

After a few years Parvin gave birth to a daughter, Nita, who is now 14. Motherhood has brought powerful emotions to the surface. "When I look at Nita, I can't believe my mother forced me into a marriage that set us against each other like that. I feel angry and sad. How could she do that to her daughter?" Parvin is determined to ensure Nita has more life choices. "I want her to go to college, and to decide who and when she marries."

Nita is currently studying at a church-run school, where she is teased by her classmates because of her unusual family setup – another reason Parvin wants the tradition abolished. But, with the pull of Bangladesh's dynamic cities causing an exodus of young people from ethnic areas, she also wants her daughter to be proud of her Mandi heritage. "Mandi women have run this tribe for hundreds of years," she says. "Now it's up to Nita's generation to run it better."