Princes William, Andrew, Edward and his wife Sophie have land in Scotland as Prince Harry travels separately to see the Queen.
The four royals' plane touched down at Aberdeen Airport before they were whisked off in cars towards Balmoral.
The journey from the airport to the royal estate is expected to take an hour by car.
Prince Harry is also said to be on his way to see the monarch.
It was originally reported Meghan, Duchess of Sussex, was travelling with her husband to be by the Queen's side.
But now it has been confirmed Meghan will remain in London, but will not attend the WellChild awards - the event the Sussexes were due to attend this evening.
The Duchess could potentially join Harry in Scotland at a later date, the source said, following the change of plan.
The senior royals will join Prince Charles and Princess Anne who are already with their mother.
The family has dropped everything to rush to the royal estate after Buckingham Palace released a concerning statement this morning.
The Royal spokesman said: "Following further evaluation this morning, the Queen's doctors are concerned for Her Majesty's health and have recommended she remain under medical supervision.
"The Queen remains comfortable and at Balmoral."
Harry and Meghan were due to attend the WellChild Awards - which celebrates the inspirational qualities of the country's seriously ill children and those who go that extra mile to make a difference to their lives - this evening.
Harry was expected to make a speech at the ceremony this evening, but now he and Meghan are making their way to Scotland.
The Duke and Duchess of Sussex were on the last leg of their UK tour and have not yet seen the Queen or any of the royals.
