When you in breed two siblings....any faults of either are doubled.
Past reading I have done...has suggested no closer than aunt/nephew or uncle/niece.
To answer your original question...yes, they can mate, there isn't a rule against it....but you'd better be very sure of the dog's faults or genetic history. They are getting a double dose of all of it.
__________________ Deb, Reese, Reggie, Frazier, Libby, Sidney, & Bodie |