1 Harry Is Fifth in Line for the Throne, Behind William’s Children
This might seem an odd rule, seeing as though Harry is fifth in line for the throne, behind his brother, and his three children, Prince George, Princess Charlotte, and Prince Louis. However, there is a good reason why the brothers shouldn’t travel together.  2 But If Something Happened to William, He Would Have to Act As Regent for the Minor
If something happened to King Charles and Prince William, then his children would be next in line for the throne. However, because they are all under the age of 18, Prince Harry would have to act as regent for George until he was old enough to officially take the throne.  3 This Is Likely Why Harry Didn’t Fly with His Relatives
If Harry and William were both killed on a plane together, it would be a constitutional crisis. Technically, this would leave Prince Andrew as regent, followed by Beatrice. This could explain why Prince Harry did not board a flight out of RAF Northolt with his brother, Prince Andrew, Prince Edward, and Sophie of Wessex. 4 There Is a “Strong Constitutional Reason” They Don’t Fly Together

“There is a strong constitutional reason that William and Harry do not fly together – and that is to do with the line of succession,” constitutional expert and royal author Brian Hoey told The Sun. “Royal aides have to consider everything, and this is very likely to have been a factor.” 5 They Likely Won’t Fly Together Again
“While events happened very quickly, they have had years to plan for all eventualities. The speed of events caught everyone off-guard and could have affected travel plans – but it still would have been unwise for William and Harry to fly together,” Hoey continued. “I don’t think we will see them fly together again for constitutional reasons.“ae0fcc31ae342fd3a1346ebb1f342fcb