If Harry is to be believed, William threw a hissy fit over the founding of Invictus, so it's a bit much to expect the family to be supportive under the circumstances of their continuing estrangement.
Also, the military patronages of the RF are largely performative, and Charles's role as Head of the Armed Forces role is purely ceremonial. They are all colonels of one regiment or another, but no one believes that Colonel Kate, Colonel Camilla, Colonel Edward etc would ever make decisions about troop deployments, let alone go to war.
As far as I know, Charles can't even declare war without government advice based on parliamentary approval.
There was a photo that delighted Twitter at the Queen's funeral when Harry wore his morning suit, while the others were weighed down by their different uniforms and medals, effectively looking like a guard of honour around Harry. It was delicious irony to see the family member who served most recently surrounded by his family cosplaying soldiers. Still gives me a chuckle.
The only veterans are Harry and Andrew, and as Invictus is for actual veterans, and not those playing dress up, I think it is a very good thing indeed that the rest of the family stay out of it.
They should leave any delegations or good wishes to those who make active decisions to send men and women to the front lines, and those who have actually served.