December 19, 2005 12:00 PM

As weddings of the young and impossibly rich go, the Dec. 3 ceremony in São Paulo was decidedly low-key. Yes, bride Athina Roussel, 20, wore a $70,000 silk gown designed by Valentino, but the look was refined, and groom Alvaro Affonso de Miranda Neto, 32, wore a black suit rather than a tux. During the ceremony in a chapel on the grounds of the Oscar Americano Foundation museum, the couple exchanged brushed-gold rings (hers inset with a small diamond). At evening’s end their 1,000 guests—not a Hollywood name among them—took home favors that included Brazilian wedding confections. In the words of a guest who asked to remain anonymous, “Nothing was over the top.”

Even so, the family dynamics behind the nuptials of Aristotle Onassis’s only granddaughter—the girl who lost her troubled mother, Christina, in 1988—were complex. On the bride’s side of the aisle were Athina’s half-sister Sandrine, 18, a few aunts and cousins and chums from her school in Switzerland. But neither father Thierry Roussel, 52, stepmother Gaby, nor half-siblings Erik (a few months Athina’s junior) or Johaana, 14, were present. The groom’s father, businessman Ricardo Miranda, walked the bride down the aisle. “She never mentioned her father and we didn’t ask,” said groomsman Constantinos Kotronakis, an honorary Greek consul to Brazil.

By marrying Doda, as he is called, a Brazilian show jumper and Olympic bronze medalist whom she met at a stable in Brussels when she was 17, Athina was, in many ways, declaring her independence. Living with Doda since March 2003, she “seems really settled here,” in the words of a clerk who works in a store near their luxury building in the exclusive Vila Nova Conceição district. Close to Viviane, 5, Doda’s daughter with model Sibele Dorsa (and a member of the wedding party), Athina also has a tight group of friends at the Paulista Equestrian Club, where she and Doda train for up to five hours daily.

While Athina has loved riding since she was a child, friends note that the passion for show jumping that she shares with Doda has kept her grounded: Both hope to compete in the 2008 Olympics. “Unlike her poor mother, who had so much trouble maintaining her equilibrium,” said an Onassis insider, “Athina goes to the health club, she jogs—she and Doda go out, but they’re not nightlife addicts.”

Not that they weren’t ready to party until the sun came up after their 9 p.m. nuptials (he cried, she didn’t). In a Brazilian-themed reception that featured Veuve Clicquot, masses of hydrangeas and bossa nova, the two whirled on the dance floor to a 40-piece orchestra. Doda danced with Viviane, São Paulo socialites boogied to “Stayin’ Alive” and the bridal couple reportedly left around 6 a.m. Even as Athina was celebrating her new life as Senhora Miranda, however, there were reminders that she would always be an Onassis: Late into the night, helicopters buzzed above the reception, and hundreds of security guards patrolled the verdant museum grounds to keep the paparazzi at bay.

Michelle Green. Flavia Celidonio, Elizabeth Johnson, Seamus Mirodan, Ciara O’Sullivan and Linda Trischitta in São Paulo, Toulah Vlahou and Dimitra Gaidatzi in Athens and Pete Norman in London