"Line breeding" or inbreeding is very common practice for poultry keepers and especially exhibition breeders. that is how you keep the most desireable traits in your flock. as long as egg production, fertility, unwanted mutations, and birth defects are not a problem, there is no need to outcross. outcrossing can blur the line you have drawn with your breeding stock even if an exceptional bird was used.
I personally wouldn't let them do that, or at least if they do i wouldn't let them hatch out any ducklings. Most breeders spend time looking for good breeding stock that will add biological diversity. When you let them breed siblings and parents the odds of getting deaths in the egg and mutations among ducklings increases. And I'm sure you don't want to hatch out some and have them die or be disfigured. an example would be the impressive bred horses. many breeders wanted to keep there stocky muscular build and they ended up breeding siblings/parents etc. the result is that now most impressive bred horses of nh or hh. which is a neurological disorder.
From observing the wild mallards at my old home for many years, it was not an uncommon thing for them to pair up with siblings. However, bear in mind that in the wild there are more drakes to be possible fathers to the ducklings than in a small, localized flock. I really don't believe that you ought to have any serious trouble, though, especially if you have more than one drake fathering the brood. But that's just my opinion.
It would seem to me you are probably safe at the very least for a generation or two. The example of inbreeding problems KansasKid gave come from many generations of trying to perfect a line. Ducks in the wild do tend to stay in their own flocks and like you were wondering, inbreeding does occur frequently. If the rates of bad mutations happened too frequently as a result of one or two generations of inbreeding, wild flocks wouldn't survive.
I agree with Born a duck lover about the multiple fathers in the flocking breeds like the mallards. But one thing is that after fully-feathered, the drake will run off the ducks and keep the hen for himself. My last batch that was hatched out sometime at the end of june is now fully feathered and usually stays right with there mother. However lately the drake is getting a little more aggressive and will "herd them" away from the hen and then the two will stay on the other side of our pond until nightfall. But that doesn't necessarily rule out the fact that the brothers and sisters still can't breed, so i will eventually get some more hens and drakes in the spring. It is totally up to you how you let them go about things, at the very least i would get an outside hen and or drake.
Actually most serious breeders line breed. Blind outcrosses are the absolute surest way to screw up a strain.
I've bred poultry for nearly 50 years & dogs for over 30. I line breed both.
Do a search here for line breeding. There are several extensive threads covering this practice.