The Queen has been slower with honours than her predecessors, and she seems to have perhaps gotten a little more so as her reign has advanced. Not just for her family and in-laws either. GCVOs for governors-general seem to have dried up by the 1990s or so (although this is perhaps a result of her reduced travel). And her current private secretary didn't get his KCVO for four years, even though his predecessors received it immediately on (or just before) appointment.
Andrew and Edward started out with CVOs and were promoted twice, and Harry started with a KCVO. In previous generations they would have gotten a GCVO quite early on. It must be a conscientious decision to reduce some of the honorific trappings of the royal family, because she's made several such decisions. Edward getting an earldom for the time being also fits into that. I believe Hugo Vickers wrote in Royal Orders that the Duchess of Kent was seen as having to wait quite a long time for her GCVO compared to previous royal wives.
I do think it's odd that neither her husband nor eldest son hold the Royal Victorian Order in any grade (the Duke of Edinburgh eventually received the Royal Victorian Chain). She seems to max out at three British knighthoods for her family, so it fits her practice, but their absence from an order given for personal service to the monarch is still striking given that they're perhaps the two people who have done the most work on her behalf at this point.