She said once that they only met when the papers already have talked about "them" and they they laughed together at it. But on the other hand she is reported to be part of the Glosse Posse. Contradicting reports, no? I don't know what is the true. In any case, I don't think they were ever close and if they were, they are certainly not anymore, I think. So maybe they weren't even invited? thought Isabella was at the royal wedding... Even if they were, I cannot imagine Kate & William going to a 4-day wedding in Africa while she's 5 months pregnant. And Kate never seemed very fond of Isabella, IMO, and Sam not being invited to the royal wedding shows the couple are not friends, at all. Kate and William's absence is not something that surprises me.
But then there's Harry. Him not being invited is a bit hard to believe IMO, for various reasons: 1st, wasn't Harry friends with Sam? even if not close friends, I think they were reported to be friends. 2nd, even if he was not in the list guest I think it wouldn't take much trouble to include another guest. 3rd, he and Cressida may be in the beginning of their relationship, but I think they are not in that phase of "seing if it works, way too early to meet my family, even if casually at an event" either. Harry already knows Cressida's mother apparentely and I suppose he knows the Calthorpes for ages.
But he could have been invited and decided not to attend. I thought it was for his work but he seems no to be back yet. Maybe he did not wanted to bring even more attention to him and Cressida, which makes me question how serious he and Cressida are, i mean, if they are serious sooner or later they'll be at weddings et all and they know everyone's attention is on them right now. Or maybe he prefered not to go because Hello! is there.