Saja El-Khawas, six, had been in critical condition after she was pulled from the sea at Shoreham on Tuesday and died today in hospital.
Sara el Khawas, 14, also died alongside her parents, Hassan el Khawas, 55, and Zeina Alayn, 37, during a trip to West Sussex.
The Palestinian embassy confirmed Mr el Khawas was a Palestinian refugee from the Rashidieh Camp in Lebanon.
It is believed the family travelled from their home in west London to visit Shoreham and entered the water from the beach near Shoreham Fort at around 2pm to go for a swim before they got into difficulty.
Several ambulances, two air ambulances, a HM Coastguard helicopter, an RNLI lifeboat, and rescue teams were all part of a major response at the scene, where the four were recovered from the water.
On Wednesday, Sussex Police said their investigation remains open, but after a review of CCTV footage and initial witness accounts, it is currently thought the deaths were a tragic accident, with no third-party involvement.
Chief Superintendent James Collis said: "The events of Tuesday afternoon are a heartbreaking tragedy which has devastated a family and their friends and had a profound impact on the Shoreham community."
Ellen Wilkinson School for Girls posted a tribute to Year 8 student Sara following the "tragic incident".
The school, in the London borough of Ealing, said: "Sara was a sweet girl, well-liked by staff and students and she will be greatly missed.
"Our thoughts and heartfelt condolences are with her family, friends, and all those affected.
"This news will be deeply upsetting for many students, families and staff."
The Palestinian embassy, in a statement on Facebook, said it was "closely monitoring the incident".
Adnan Hmidan, chairman of the Palestinian Forum in Britain, paid tribute to the family, although his spelling of their name differed from that used by Sussex Police.
He said: "The Palestinian, Arab and wider community in Britain, along with the many friends and supporters of our community, have been deeply shocked and saddened by the tragic incident involving the Khawwas family during a day trip off the coast of Sussex."
Prime Minister Andy Burnham warned of the dangers of open water swimming after the incident, which he described as "a terrible family tragedy".
This breaking news story is being updated and more details will be published shortly.
Please refresh the page for the fullest version.
You can receive breaking news alerts on a smartphone or tablet via the Sky News app. You can also follow us on WhatsApp and subscribe to our YouTube channel to keep up with the latest news.
Earlier, police said two teenage boys were severely injured, and a third person was also taken to hospital with the extent of their injuries unclear.
Police were called to Brinell high school in the town of Fagersta, northwest of the capital Stockholm, on Friday.
The suspect, who was shot, has been arrested and is being held in custody, authorities said.
A police spokesperson said: "We received an alarm at 2.06pm (1.06pm UK time) about suspected deadly violence and sent all available resources to the school."
The spokesperson said there was no longer any danger to the public.
Public broadcaster STV, citing unnamed sources, reported that the person arrested was a former student at the school, who had a previous conviction for assault.
A student at the school said he had seen a victim with a stab wound in the arm, saying: "Then people called the police and we just all ran from there because we were afraid."
Several other schools and public buildings in Fagersta were also placed in lockdown after the sword attack took place.
Pelle Vamstad, a police spokesman, told the TT news agency that authorities would be looking into whether camera footage could be used in the preliminary investigation.
Sweden's Prime Minister Ulf Kristersson, in an online post, said a "serious incident" happened in Fagersta "right after the summer break".
"There are reports of injured people. We do not yet know what lies behind the act, but we know that the police are working very intensively," he added. "Our thoughts are with all those affected."
Read more from Sky News:
British cyclist to compete in race after brother's death
Five missing after fire at restaurant with apartments
Brinell high school has around 450 students aged 16 to 20.
The attack comes a year after a school shooting in Orebro, also in central Sweden, when a gunman killed 10 people before turning the weapon on himself, in the worst mass shooting in Swedish history.
Midfielder Leandro Paredes has been given the longest ban - of 10 matches - and a £66,000 fine by FIFA's disciplinary committee.
Defender Nahuel Molina was handed a seven-game suspension and fined £66,000, while Thiago Almada was given a one-match ban and a £22,000 fine.
Argentina coach Roberto Ayala was banned for three matches and fined the same amount.
Read more:
Hunt for 'Chucky-style' masked man who chased woman
The hackers blackmailing a video game giant
Spain's Gavi was ordered to serve a one-match ban and fined £22,000.
Argentina's FA (AFA) has also been fined £235,000 over the displaying of a banner following the semi-final win over England, which read "Las Malvinas son Argentinas", translating to "The Falklands Islands are Argentinian".
Only £89,000 of that is an immediate direct fine to FIFA, with £73,000 suspended and another £73,000 Argentina will spend on its own anti-discrimination work.
The fine also covered breaches by Argentina supporters of FIFA's anti-discrimination regulations at other games during the tournament.
For the discrimination offences, the AFA has been ordered to play its next home match at 50 per cent capacity, with another 50 per cent restriction at a further match suspended.
The Falkland Islands is a UK overseas territory with islanders voting overwhelmingly to remain so in 2013.
In 1982, Argentina's forces invaded the islands but were forced to surrender two months later.
The banner was condemned by UK politicians at the time.
Business Secretary Peter Kyle also described the banner as "entirely inappropriate".
AFA said its legal department would "request from FIFA the grounds for the rulings that are reviewable in order to file appeals before the Appeals Committee of the governing body of world football, leaving in the future the possibility of appealing to the CAS as a last resort".
There was a police presence at Friday's funeral masses for Joe Carthy, 15, and Alex McCarthy, 17.
They were among five teenagers killed when the vehicle they were in drove the wrong way along the M9 and collided with a car carrying three sisters - Ella Hendricken and Niamh and Alma Kinsella - and a young boy.
They remain in hospital with serious injuries.
The other three teenagers who died in the County Kildare crash were Jack Kennedy, Kamil Pustkowski, both 17, and Jeremy O'Brien, 18.
A vigil is being held on Friday evening in support of the Kinsella family, who were travelling to Dublin Airport for a wedding.
The crash has ignited national debate in Ireland about dangerous driving and the regulation of social media, after it emerged some of the teenagers had posted content of cars being stolen and driven illegally.
Joe Carthy's funeral began in Athy, Co Kildare, where police were highly visible in the town amid concern about the risk of public disorder.
The coffin arrived at the church on a white horse-drawn carriage.
Mourners followed the coffin inside, wearing white T-shirts depicting an image of the boy, while others raised framed photographs.
The priest extended his "sincere sympathy to Joe's family", adding: "We gather with sorrow in our hearts."
National police and security officers maintained checkpoints on the road into town, and police were stationed around the church.
Outside the service, a screen displayed pictures and videos while music played, and white flowers were placed around the church's entrance.
See more from Sky News:
Hiker wrongly reported alive
Sussexes to return to UK
There were similar scenes at the funeral for Alex McCarthy in Askea, Co Carlow, with police present and a horse-drawn carriage bringing the coffin to church.
Father Tom Little told mourners they had come together "with heavy hearts and a deep sense of sadness" as he said prayers would remember "families on both sides of the accident".
"Conscious that lives have been lost, others have been injured, and families have been left to carry the grief that is so difficult to put into words," he said.
"A tragic accident can happen in seconds and the result of it can last a lifetime. These types of accidents bring a deep sense of shock, anger, questions, regret, and even a sense of disbelief."
Elsewhere in Carlow, the vigil for the three sisters and the boy will be held on Friday night.
A fundraiser for the family has raised £548,000, amid an outpouring of public support.
The crash has also shifted attention onto Ireland's child and family agency, Tusla, after the organisation confirmed one of the teenagers was in its care and two other boys were "known" to it.
Government and opposition politicians have criticised social media companies for failing to quickly remove content depicting dangerous driving.
TikTok declined to appear before a parliamentary committee probing content depicting unsafe driving on social media.
Irish justice minister Jim O'Callaghan committed to providing An Garda Síochána with "whatever resources they require" to deal with dangerous driving, including introducing pursuit-specific training for the first time.
It emerged in the wake of the crash that gardai had been instructed not to pursue the vehicle after it began deliberately driving the wrong direction down the motorway.
Paul Price was speaking at a ceremony in central London for the inaugural National Day for Victims and Survivors of Terrorism.
He was left with catastrophic injuries and spent months in hospital after the 2017 Manchester bombing, in which his partner Elaine McIver was killed along with 21 other people.
Campaigners have also claimed it is a "national scandal" that terror attack victims are still being "mistreated and neglected".
The day focused on remembering and recognising those impacted by terrorism, and Andy Burnham also spoke at the ceremony to honour victims and survivors.
Mr Price told the event that "terrorism does not end when the emergency services leave the scene".
He said: "It continues for years in hospital wards, in empty chairs at the dinner table, in children growing up without parents, and in survivors who must relearn how to inhabit their own bodies".
Thelma Stober, who lost her leg in the July 7 London bombings in 2005, also suggested better medical and financial support for victims was needed.
She suffered from headaches for years which were initially diagnosed as migraines, before shrapnel was found during a brain scan in 2019.
Ms Stober said: "I'm doing it privately now, I can, I am in the position where I can do that - but what about the many others who are going through the same or similar?
"What about the families who lost loved ones, what support do they have?"
The group Survivors Against Terror (SAT), founded in 2018, has put forward a "survivors charter" based on support offered in other countries.
It claims the state is failing in areas such as financial assistance, compensation, access to legal advice and recognition of victims.
Brendan Cox, husband of murdered MP Jo Cox, co-authored a report by the group.
The report claims the Ministry of Justice has "delivered no substantial progress" and that "survivors of attacks are having their lives and recovery made harder".
"It remains a national scandal that survivors of attacks are being mistreated and neglected in this way."
Mr Burnham said the government would look at reforming the Criminal Injuries Compensation Authority (CICA) - where terrorism victims are required to seek compensation - and access to legal assistance.
Read more from Sky News:
Killer jailed for knife murder
Driver jailed for killing great-grandfather
The prime minister told attendees: "We will fight for you. The government I lead will fight for you too. And I'm sure all of the politicians in the room would say the same."
A government spokesperson said victims deserve access to "financial compensation and support".
They highlighted that the CICA paid out more than £180m last year to violent crime victims, including terrorism, and pointed to a £550m investment in wider victim support services.
A dedicated support service has also been launched for terror attack survivors, which provides specialist assistance and help with the compensation process.