Arsalaan Habib, four, his brother Adnan, 10, and their aunt, Bushra Tazarib, 32, died on the M6.
The boys' mother Parveen, 38, who was driving, and their 17-year-old sister Misbah survived.
All five were on their way to Bolton - where the boxer lives - when the accident happened.
The bride, Rabia Khan, is the former world champion's second cousin and he was at the ceremony when the news came through, his father Shah Khan confirmed today.
Mr Khan said the family were from the groom's side and that Amir and the rest of the family were very upset to learn of the accident.
He said: "It shocked everybody. The family is trying to support them."
The boxer said: "My heart goes out to the families of those involved in this terrible accident. No one could believe it. One minute everyone at the wedding was joyous, the next it had turned into despair."
The boys and their aunt died after the car they were travelling in crashed and overturned on the M6 near Stoke-on-Trent at about 11.25am.
All three were from High Wycombe in Buckinghamshire, Staffordshire Police said.
The boys' mother - who was driving the red Ford Focus - and a female relative who was a fellow passenger were both injured, and were taken to hospital.
They were discharged from the University of North Staffordshire Hospital yesterday.
A Staffordshire Police spokesman said: "Specially trained liaison officers have been appointed to support them at this incredibly difficult time for the family."
Chief Superintendent Martin Evans, of the Central Motorway Police Group (CMPG), described the incident as "unimaginably sad and tragic".
Meanwhile, officers have issued a public appeal for more information to try and piece together what might have caused the accident, which is believed to have involved one vehicle.
The incident led police to close the motorway in both directions for three hours from junction 14 at Stafford, to junction 15 at Stoke.
At the crash scene, medics were supported by two air ambulances, and colleagues from the fire and police services.
West Midlands Ambulance Service spokesman John Hawker said: "Crews arrived to find a single vehicle that had crashed and overturned.
"Two children and an adult that were travelling in the vehicle suffered serious injuries.
"Unfortunately, despite the best efforts of ambulance crews, nothing could be done to save the two boys and one woman, and they were confirmed dead at the scene."