Shelley Rothman-Benhaim said her brother Howie Chaim Rotman suffered multiple stab wounds when two Palestinian cousins stormed into his local temple wielding meat cleavers, knives and a gun.
The attackers killed four rabbis before being slain themselves in a police shootout. A police officer later died of his injuries.
The attack, the deadliest in the country since 2008, left the Israeli government vowing retaliation and drew the ire of world leaders including Prime Minister Stephen Harper.
Rothman-Benhaim said that her brother underwent surgeries on his head, eyes and arms Tuesday and is currently in a medically induced coma in a Jerusalem hospital.
The 54-year-old father of 10 welcomed his first grandchild just last month, she said, adding Rotman's prognosis likely won't be known for the next two or three days.
"It's very, very serious," Rothman-Benhaim said in a telephone interview from her home in Montreal. "People are praying for him around the clock, and that's what we're asking people to do is to pray."
Rotman was born in Toronto, but relocated to Israel in his early 20's in order to more fully embrace his faith.
The man known as Howie Rothman in Canada reverted to his hebrew name of Chaim Rotman once established in Jerusalem, where his sister said he married and worked as an auditor for the Israeli government.
She said his move was driven by his love of Israel and desire to study more Judaism and follow commandments that can only be accomplished in the Holy Land.
The synagogue where the attack unfolded was just metres from Rotman's home, Rothman-Benhaim said, adding most of the victims were his fellow regular worshippers.
The government released a photo of a meat cleaver it said came from the crime scene in the west Jerusalem neighbourhood of Har Nof. Government video showed blood-soaked prayer books and prayer shawls in the synagogue. A pair of glasses lay under a table, and thick streaks of blood smeared the floor.
"I saw people lying on the floor, blood everywhere," said Yosef Posternak, who was at the synagogue in the quiet neighbourhood that has a large community of English-speaking immigrants.
"People were trying to fight with (the attackers) but they didn't have much of a chance," Posternak told Israel Radio.
The U.S.-born victims were identified as Moshe Twersky, 59, Aryeh Kupinsky, 43, and Kalman Levine, 55. The Israeli Foreign Ministry said the British man was Avraham Goldberg, 68, who immigrated to Israel in 1993.
It described the four as "rabbis," an honorific title in the ultra-Orthodox world given to men who are considered pious and learned.
Twersky, a native of Boston, was the head of the Toras Moshe Yeshiva, a seminary for English-speaking students. He was the son of Rabbi Isador Twersky, founder of Harvard University's Center for Jewish Studies, and a grandson of Rabbi Joseph B. Soloveitchik, a luminary in the world of modern Orthodox Jewry.
Thousands of people attended a joint funeral for Kupinsky, Levine and Goldberg before sundown — held outside the synagogue where they were killed.
Canada's Department of Foreign Affairs did not identify Rotman by name, but said they were offering support to a dual Canadian-Israeli citizen.
"We express our condolences to the victims and wish a speedy and full recovery to those injured in today's deplorable terrorist attack," spokesman Francois Lasalle said in an email.
Politicians the world over spoke out against the attack, which comes at a time of escalating violence across Israel.
Harper took to social media to offer support.
"Canada condemns the barbaric act of terror against a synagogue in West Jerusalem," Harper said in a tweet. "Our thoughts and prayers are with the people of Israel."
U.S. President Barack Obama said there can be no justification for such attacks. Obama urged Israeli and Palestinian leaders and ordinary citizens to "work co-operatively to lower tensions, reject violence and seek a path forward towards peace."
Palestinian President Mahmoud Abbas condemned the attack, the first time he has done so in the wave of deadly violence against Israelis. But he also called for an end to Israeli "provocations" surrounding Jerusalem's shrines that are sacred to both Muslims and Jews.
But his words weren't enough for Israeli Prime Minister Benjamin Netanyahu, who accused Abbas of inciting recent violence.
Netanyahu ordered the demolitions of the attackers' homes.
In recent weeks, Jerusalem has seen its worst sustained bout of violence since a Palestinian uprising a decade ago. Palestinian assailants have carried out a pair of deadly attacks by ramming their cars into crowded train stations, while a gunman shot and seriously wounded a Jewish activist who has campaigned for greater access to a holy site.
The hilltop compound, in Jerusalem's Old City, has been at the heart of the tensions. It is revered by Jews as the Temple Mount, the site of the ancient Hebrew temples. For Muslims, it is the Noble Sanctuary, home to the Al-Aqsa Mosque and the iconic gold-topped Dome of the Rock.
Under a longstanding arrangement, Jews are permitted to visit but not to pray. A growing number of visits by Jewish worshippers, many who seek the right to pray there, has drawn Muslim accusations that Israel is secretly trying to take over the site and sparked violent clashes between young Palestinians and Israeli police.
Netanyahu has repeatedly said he will not change that arrangement, but the violence has spread beyond Jerusalem, with deadly stabbings in Tel Aviv and the West Bank last week, while the fatal shooting of a young Arab protester in northern Israel by police — apparently as he was walking away from an officer — has added to the tensions.
Police identified the synagogue attackers as Ghassan and Oday Abu Jamal, cousins from the Jabal Mukaber neighbourhood in east Jerusalem.
Mohammed Zahaikeh, a social activist in Jabal Mukaber, said he did not know whether the cousins were politically active. He said Ghassan was 27, married with two young children and worked in a clothing store. Oday, 21, was not married and was an interior decorator. He said both men were quiet, and residents were surprised by the attack.
The attack was the deadliest in Jerusalem since a Palestinian assailant killed eight students at a Jewish seminary in March 2008.
With Files from the Associated Press