could anybody tell if there are any risks in breeding half brother to half sister

The risks are doubling up on producing whatever hereditary faults they are carrying!  You need to really KNOW each of the dogs behind sire and dam for several generations to do this which any confidence. 
But then it's equally important to KNOW the ancestors when outcrossing.  It's a common fallacy to think that if sire and dam aren't related then they will produce healthy puppies.

If the common patent has a particularly undesireable trait then it is more likely to also appear in the puppies.  For this reason knowledgeable breeders would be doing this sort of breeding if the common parent were outstanding, and would be checking even more carefully into his or her background regarding health temperament and faults.

It is also a very good way to quickly ascertain if there are any underlying recessive characteristics to look out for when using a new bloodline quickly telling the breeder if they should continue with the new line, or breed away from it.

Hi Guest,

Of course there are risks - there are in any breeding - *BUT* the closer the relationship of the breeding pair the more magnified any faults, failings, inherited defects are as well as virtues - it is not the type of mating anyone carries out without intricate and detailed knowledge of the lines which they are doubling up on as well as the individual dog and bitch themselves.  

With due respect, if you need to ask this question it suggests that it is not an avenue which you should consider taking.

Regards, Teri