Relatives of the Boston University junior from Salisbury who was critically injured in a New Zealand crash that killed three other students are agonizing as they await an update on her condition after she underwent brain surgery over the weekend, her grandmother said on Sunday.

“Hellish,’’ said Annette Theriault, 74, of Beverly when asked to describe her family’s wait for news of Margaret Theriault, 21. “Hellish, that’s all. It’s not a very good Mother’s Day… . You’ve just got to roll with the punches.’’

Theriault, a retired nurse, said brain injuries are very unpredictable.

But, she said, “I’ve seen miracles and I’ve seen the worst of it. I’m just hoping a miracle happens.’’

A spokeswoman for Waikato Hospital in New Zealand would only say the student remained in critical condition there.

In a statement released by the hospital on Monday morning New Zealand time, Margaret’s parents, Todd and Deb Theriault, said their thoughts and prayers were with the families of the students killed in the crash.

They said they had arrived in New Zealand to be with their daughter, called Meg.

“We’re here now and we’re supporting our daughter for as long as it takes,’’ they said in the statement. “She is a fit and stubborn young woman and we know she is getting the best care.’’

Annette Theriault said the entire family is grief-stricken.

“Well, we’ve cried a lot,’’ she said. “And we’re waiting and waiting. That’s all we can do, and you pray.’’

Margaret Theriault’s aunt, Terri Killam, 52, of Danvers, was at the home of Annette, her mother, on Sunday, where family members were gathered anxiously awaiting news.

“All we can do is ask you to pray for us,’’ Killam said. “That’s the only thing you can ask for.’’