The legendary All Black died on November 18, 2015 of a heart attack following a long battle with kidney failure.
Lomu's third wife Nadene - who he had two sons with - is in charge of his grave at the Manukau Memorial Gardens.
His burial plot - like fellow Pasifika players Peter Fatialofa, and Dylan Mika - reflects the number on his All Blacks jersey: in his case 11.
"We're a bit suspicious about the timing of the unveiling – we just found out like everybody else on Thursday," his brother John Lomu told the Herald on Sunday.
"But I am glad it's done and even if Mum can't make it Jonah can finally rest in peace."
Lomu's mother Hepi Lomu was already booked on a flight to Fiji on Friday and was unable to change her airfares in time to attend the unveiling.
John Lomu said he understood from a conversation he had with Nadene that the headstone was completed earlier this year.
"We were talking at the start of the year but somewhere along the lines of communication broke down," he said.
"She was putting it out there the headstone was finished.
"But there was no headstone and she asked me if I would contribute to it."
Lomu's close friend and personal trainer Henry Vaeoso suspected the unveiling was timed to coincide with the French Test series against the All Blacks.
"It wouldn't surprise me if Nadene has planned this while the French team is out here," he told the Herald on Sunday.
Vaeoso said he was deeply concerned about the epitaph on the headstone.
He claimed it was written without consultation with members of the Lomu family.
"I hope if there is anything on the stone it would be Jonah's mum, his siblings, Nadene and the two boys.
"It better not have Nadene's parents on it because it would be a big insult to the Lomus ... I know the boys will dig it up."
Vaeoso said he wanted his friend's name to be remembered "for the good things".
"And not be tainted with this family who has come out of nowhere and told they are the only ones that matter," he said.
Vaeoso will attend tomorrow's service with Lomu's "tight five" - Michael Jones, John Hart, Eroni Clarke, Junior Tonu'u and John Lomu, all loyal friends who have stood by him from "day one".