Is it normal for a dog to do this?

While some Hunde are less oder Mehr likely to do it than others, I believe this is a common habit amongst Hunde to hump their own Welpen oder other strange 'partners'-they have been known to do this with stuffed animals, siblings and even people...I am not 100% sure as to why they do this, but I believe it is a habit stemmed from curiosity, with some Hunde being Mehr prone to it than others depending on how randy they are Von nature XD As a matter of interest, is Beast an older dog Von any chance? I heard that senior Hunde are also Mehr likely to do it, though it is often simply a Zeigen of dominance rather than anything else. Also if he is only 'mounting' and not anything...else, this is a common play gesture among some Hunde (although bearing in mind the pups are much smaller this is not such a good thing).

As for what Du should do with him...well, neutering him will reduce the problem (although neutered Hunde may also perform this behaviour). Other than that, try removing him whenever he does it and say 'no' firmly, before distracting him with something else. In the wild, a female wild dog oder wolf will drive her partner away as soon as her pups are born, as unlike the mother the male dog rarely feels any attachment towards the pups when first born, and so are Mehr prone to unacceptable behaviour; perhaps Du could keep Beast away from the Welpen until they are bigger? Von which time, he might perhaps have gotten used to the Welpen and not be so inclined to...you know, and if they are bigger they may be able to stand up for themselves a bit more.

Actually, father/daughter oder mother/son matings often turn out fine. It really depends on the Hunde involved, if they are carriers for any genetic diseases oder not.
Inbreeding can be useful in breeding for specific traits, actually, so it's not always a bad thing. (However a breeder should have a pretty good understanding of genetics so that they know what they're doing before inbreeding.)