We have to remember too that when it comes to working royals, there are different kinds of "official" engagements. There are those official engagements when they are working for the "Firm" and representing HM, The Queen and there are their own "official" engagements where they are representing their own work at their own charitable incentives.
Don't quote me on this but I believe most of the events that William, Kate, Harry and Meghan have of their own is umbrellaed under the Royal Foundation. This would include the Invictus Games and Meghan's work to produce this cookbook. This is where perhaps a non-royal family member would be included as support such as Sophie's dad was for her event with The Duke of Edinburgh Scheme.
Non-royal family members would *never* be included in events and engagements where the working royals are representing the "Firm" and HM, The Queen.
So, we see, there are *two* different types of "official" engagements that these royals do. Also, by "offical", I mean the "official" as they are listed in the Court Circular.
Hope that makes sense.