Nadene Lomu said the rift cannot get in the way of how Lomu's sons Brayley and Dhyreille were dealing with losing their dad.
"If that's how she wants Jonah's grave to be, we can't even leave anything on there for him."
Asked about a family divide, Nadene Lomu said her and Jonah "kept to ourselves as a family.
"Whatever differences of opinions there are with Jonah's family... all differences aside, it is a place of rest. It is a sacred place."
Earlier today, Nadene Lomu posted an anguished social media call for help, in which she asked people to to leave Jonah Lomu's grave alone.
Jonah Lomu died two months ago of cardiac arrest after years of chronic kidney illness, just after returning from the Rugby World Cup.
Nadene Lomu said she and their boys had made regular visits to Jonah Lomu's resting place which the children decorated with flowers and other adornments.
But return visits to the grave at the Manukau Memorial Gardens in South Auckland have left the family stunned to find the burial site cleared back to bare earth.
Today the family arrived to find the burial site cleared again, and spent the morning replacing flowers. "The boys were playing in the dirt, putting the flowers in how they wanted."
As they were leaving, Hepi Lomu arrived.
"As we were leaving, we said 'hi' to his mum. I spoke to her about what had been happening and how people had been taking things.
That was when, according to Nadene Lomu, her mother-in-law said "it was her right to do what she would with it".
Nadene Lomu said she returned to the freshly-decorated grave and removed the decorations and artificial flowers her and Jonah Lomu's sons had just placed.
"I took it all out. I had to take it out or she said she would. I don't know why. I've got the flowers sitting on the back seat of the car.
"I didn't want to get into an argument."
The Herald is seeking comment from Hepi Lomu.
On Facebook early today, Nadene Lomu wrote how "it saddens me that our sons things they choose for you keep being taken off & taken away from yr place of rest".
Nadene Lomu told the Herald her Facebook post was driven by anguish over the boys' memorial to their father being removed.
She told the Herald: "Every time we've turned up there have been things that have constantly been taken. We keep taking more and more things.
"To come one day and the candy canes have gone, then the windmills have gone. A letter my son's class did for him... that's gone."
She says the visits to the grave and leaving of items there was important to "two little boys who have lost their daddy".
She woke early this morning and took to Facebook "to whoever was taking these things, it was my cry for help".
Lomu's sister Sela also said that the decorations were removed so that soil could be added to the top of the grave.
Addressing Nadene Lomu, she reportedly said on Facebook: "To my sister in law I know you would be upset that we took down your decorations on Sunday and that I had rearranged it prior to that... we had approached the cemetery people to top up our brother's grave as you would have seen it sinking below ground level..."
Auckland council manager of cemeteries Catherine Moore said she had been contacted by Nadene Lomu who had inquired about the missing decorations.
"Nadene Lomu contacted me a couple of days ago to say that over the last couple of week, several times when they have been up to visit the grave they noticed that flowers and some decorations that they had places up there had gone missing and she wondered whether we knew anything about that," she said.
Ms Moore said council records showed Nadene Lomu had the burial rights and control over that grave.
"I have spoken to our team out there and we haven't removed any of those items but also we haven't heard anything about them so I said look we can keep a closer eye on it."
She said cemetery rules allowed items to be placed on the burial mound for a fortnight before grounds' staff added top soil and grass seed, when mementos were shifted to the concrete strip at the head of the grave.
"I understand that there were a number of decorations that were up on the concrete.. and those are the items that have been removed."
Chairman of the Tongan advisory council, Melino Maka, said culture dictated that it was the role of the mother to care for the grave.
"It is a very sensitive topic but there is normally an expectation that Hepi [will look after the grave]. The way that we look at it is that in the future Nadene may marry again from a cultural and common sense perspective.
"At some stage down the line it may happen and that is why usually Hepi will have the right of passage to take over looking after Jonah's grave."
Mr Maka said he was involved in Jonah Lomu's funeral and there was a sense of issues between the sides of the family.
