It's up to them, isn't it.
And then, if they extend invitations to their father and stepmother, it's up to Charles and Camilla to decide whether William and Harry's purpose for honoring Diana will be served by their appearance at the event.
None of us really know how the two princes have (or have not) reconciled their mother's unhappiness and death with their father's second marriage. But at some point there comes the time to call a truce and (you'll forgive my Yiddish) behave like a mensch. And so far, despite some lapses of good sense on the part of Harry, I think both young men have borne the fishbowl situation with considerable grace.
I am not a fan of Camilla's participation in selecting a royal brood mare and abetting an adultery. Yet Diana is irretrievably gone, both Charles and Camilla have settled into late middle aged marriage, and Diana's legacy--her sons and, despite numerous personal issues, her humanitarianism--survives.
As for William and Harry complaining about books written by Diana associates . . . well, it is their right as sons to object to any such revelations, and I can understand their discomfort. But I think they would best serve both their mom's memory and their own privacy to become a bit more stoic, even impervious to the issue. And regarding the "Diana-ana" authors . . . well, here in the States we just recently had an example of the limits of public acceptance for publication trash. So I have faith that those who are exploiting the situation in vulgar fashion, rather than offering valid insight, ultimately will sink to the bottom of their slimey aquariums.