Taking a step back, you have two adults who are mobile, two who are not (one injured and one working for several hours), and four teenagers. You described two activities that Kate and DH did together, both of which would not have been available to you, both of which sound like fun things to do on a presumably not-cheap holiday with the clock ticking.
Without knowing much about the villa/room situation, it seems unlikely that DH and Kate are shagging, because there is presumably a substantial chance of one of the DC walking in (or hearing). They presumably aren't be at windsurfing class 8 hours a day.
The other thing that occurs to me is that it's going to be very hard for you to raise the issue in anything other than the gentlest terms. If you even remotely insinuate that you suspect that anything is not 100% above board, the holiday will be over in an instant, whatever the truth of the matter. Even if you only insinuate it to your DH.
You could maybe suggest some concrete activities, "How about we do X tomorrow, maybe Neil could join us if he doesn't have a Zoom call", but I think you're going to have to accept that if DH and Kate are sporty, they won't need you around, any more than if it was DH and Neil, with Kate working (assuming nobody is bi, etc etc). But anything much more than "Perhaps we can do X tomorrow since we can all go" is going to open the floodgates, which might not be what you want for you and your DC, even if the worst case is true.
Or, as they say in French, il est urgent d'attendre. Make some gentle suggestions of activities, maybe work on Neil a bit (perhaps he only has work for a few days), and if DH/Kate continue to not involve you, give DH both barrels when you get back home.