William, 38, could wear his military uniform, but a royal expert says it's unlikely since all eyes will be on the siblings.
"This is obviously very sensitive," Richard Fitzwilliams said. "Anything that distinguished the brothers in that way, which would be so obvious for anyone watching, would probably be avoided. The whole thing we do not want is any form of rift."
He continued, "I'd suggest it would be handled in such a way as it goes totally smoothly without a hitch, and there are no issues. My opinion is that they would do everything possible to make sure that the spotlight on this very, very important day is where it should be, and that's on truly remarkable achievements of someone with superhuman energy who did so much for Queen and country."
This is the first time Harry has seen his family since he left the royal family in March 2020. Since then, he has spoken candidly about why he uprooted his life and discussed his strained relationships with his brother and father, Prince Charles.
However, Fitzwilliams is hope that William and Harry will mend their relationship going forward. "It may be the beginning of the healing process," he said. "The Royal Family can come together as family, and it is certainly absolutely essential they present united front."
"Achieving that after the Oprah interview and so forth will be quite a feat, but I am sure that, given the circumstances, and given the fact that both Harry and William were devoted to their grandfather, there is a very very good chance of achieving it," he added.